U.S. patent number 10,930,930 [Application Number 16/289,043] was granted by the patent office on 2021-02-23 for active material, electrode, secondary battery, battery pack, and vehicle.
This patent grant is currently assigned to KABUSHIKI KAISHA TOSHIBA, Toshiba Infrastructure Systems & Solutions Corporation. The grantee listed for this patent is KABUSHIKI KAISHA TOSHIBA, Toshiba Infrastructure Systems & Solutions Corporation. Invention is credited to Yasuhiro Harada, Norio Takami, Yasunobu Yamashita, Kazuomi Yoshima.
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United States Patent |
10,930,930 |
Harada , et al. |
February 23, 2021 |
Active material, electrode, secondary battery, battery pack, and
vehicle
Abstract
According to one embodiment, an active material is provided. The
active material includes a primary particle containing a
phosphorus-containing monoclinic niobium-titanium composite oxide.
The primary particle has a concentration gradient in which a
phosphorus concentration increases from the gravity point of the
primary particle toward the surface of the primary particle.
Inventors: |
Harada; Yasuhiro (Isehara,
JP), Takami; Norio (Yokohama, JP),
Yamashita; Yasunobu (Tokyo, JP), Yoshima; Kazuomi
(Yokohama, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA TOSHIBA
Toshiba Infrastructure Systems & Solutions Corporation |
Minato-ku
Kawasaki |
N/A
N/A |
JP
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
(Minato-ku, JP)
Toshiba Infrastructure Systems & Solutions Corporation
(Kawasaki, JP)
|
Family
ID: |
69772250 |
Appl.
No.: |
16/289,043 |
Filed: |
February 28, 2019 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20200091513 A1 |
Mar 19, 2020 |
|
Foreign Application Priority Data
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|
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Sep 14, 2018 [JP] |
|
|
JP2018-172323 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/62 (20130101); H01M 50/202 (20210101); H01M
4/485 (20130101); B60L 7/10 (20130101); H01M
4/366 (20130101); H01M 10/4257 (20130101); B60L
50/64 (20190201); H01M 50/20 (20210101); H01M
10/0525 (20130101); H01M 4/131 (20130101); H01M
4/5825 (20130101); Y02T 90/12 (20130101); H01M
2220/20 (20130101); Y02T 10/70 (20130101); Y02E
60/10 (20130101); H01M 2010/4271 (20130101); Y02T
10/7072 (20130101); H01M 2004/027 (20130101) |
Current International
Class: |
H01M
4/485 (20100101); H01M 10/0525 (20100101); H01M
4/131 (20100101); H01M 4/36 (20060101); H01M
4/58 (20100101); B60L 50/64 (20190101); H01M
10/42 (20060101); B60L 7/10 (20060101); H01M
4/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2010-287496 |
|
Dec 2010 |
|
JP |
|
2011-513199 |
|
Apr 2011 |
|
JP |
|
5023239 |
|
Sep 2012 |
|
JP |
|
5925845 |
|
May 2016 |
|
JP |
|
2017-59397 |
|
Mar 2017 |
|
JP |
|
2017-168352 |
|
Sep 2017 |
|
JP |
|
2018-92955 |
|
Jun 2018 |
|
JP |
|
Other References
Madeleine Gasperin, "Affinement de la structure de
TiNb.sub.2O.sub.7 et repartition des cations", Journal of Solid
State Chemistry 53, 1984, 4 pages. cited by applicant.
|
Primary Examiner: Lynch; Victoria H
Attorney, Agent or Firm: Oblon, McClelland, Maier &
Neustadt, L.L.P.
Claims
What is claimed is:
1. An active material comprising a primary particle comprising a
phosphorus-containing monoclinic niobium-titanium composite oxide,
wherein the primary particle has a concentration gradient in which
a phosphorus concentration increases from a gravity point of the
primary particle toward a surface of the primary particle, and the
phosphorus-containing monoclinic niobium-titanium composite oxide
has an average composition represented by a general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7, where
0.ltoreq.x<1, 0.ltoreq.y<1, 0<z.ltoreq.0.5, each of
elements M1 and M2 is at least one selected from the group
consisting of V, Ta, Fe, Bi, Cr, Mo, W, B, K, Na, Mg, Al, and Si,
and the element M1 and the element M2 may be the same element or
different elements from each other.
2. The active material according to claim 1, wherein a ratio
(C2/C1) of a phosphorus concentration (C2) at a position
corresponding to 80% of a length defined from the gravity point to
the surface of the primary particle, with respect to a phosphorus
concentration (C1) at a position of the gravity point of the
primary particle is in a range of 1.05 to 100.
3. The active material according to claim 1, wherein a phosphate
compound is present at least a part of the surface of the primary
particle.
4. The active material according to claim 3, comprising a secondary
particle formed of a plurality of the primary particle, wherein the
phosphate compound is present between the plurality of the primary
particle.
5. The active material according to claim 3, wherein the phosphate
compound comprises at least one selected from the group consisting
of phosphorus oxide, iron phosphate, and potassium phosphate.
6. The active material according to claim 5, wherein the phosphate
compound comprises at least one selected from the group consisting
of iron phosphate and potassium phosphate.
7. An electrode comprising the active material according to claim
1.
8. The electrode according to claim 7, wherein the electrode
comprises an active material-containing layer comprising the active
material.
9. A secondary battery comprising: a positive electrode; a negative
electrode; and an electrolyte, wherein the negative electrode is
the electrode according to claim 7.
10. A battery pack comprising the secondary battery according to
claim 9.
11. The battery pack according to claim 10, further comprising an
external power distribution terminal; and a protective circuit.
12. The battery pack according to claim 10, comprising a plurality
of the secondary battery, wherein the plurality of the secondary
battery are electrically connected in series, in parallel, or in
combination of in series and in parallel.
13. A vehicle comprising the battery pack according to claim
10.
14. The vehicle according to claim 13, comprising a mechanism
configured to convert kinetic energy of the vehicle into
regenerative energy.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from Japanese Patent Application No. 2018-172323, filed Sep. 14,
2018, the entire contents of which are incorporated herein by
reference.
FIELD
Embodiments described herein relate generally to an active
material, an electrode, a secondary battery, a battery pack, and a
vehicle.
BACKGROUND
Recently, secondary batteries, such as a nonaqueous electrolyte
secondary battery like a lithium ion secondary battery, have been
actively researched and developed as a high energy-density battery.
The secondary batteries, such as a nonaqueous electrolyte secondary
battery, are anticipated as a power source for vehicles such as
hybrid electric automobiles, electric cars, an uninterruptible
power supply for base stations for portable telephones, or the
like. Therefore, the secondary battery is demanded to, in addition
to having a high energy density, be excellent in other performances
such as rapid charge-discharge performances and long-term
reliability, as well. For example, not only is the charging time
remarkably shortened in a secondary battery capable of rapid charge
and discharge, but the battery is also capable of improving motive
performances in vehicles such as hybrid electric automobiles, and
efficient recovery of regenerative energy of motive force.
In order to enable rapid charge/discharge, electrons and lithium
ions must be able to migrate rapidly between the positive electrode
and the negative electrode. However, when a battery using a
carbon-based negative electrode is repeatedly subjected to rapid
charge and discharge, precipitation of dendrite of metallic lithium
on the electrode may sometimes occur, raising concern of heat
generation or ignition due to internal short circuits.
In light of this, a battery using a metal composite oxide in a
negative electrode in place of a carbonaceous material has been
developed. In particular, in a battery using an oxide of titanium
in the negative electrode, rapid charge and discharge can be stably
performed. Such a battery also has a longer life than in the case
of using a carbon-based negative electrode.
However, compared to carbonaceous materials, oxides of titanium
have a higher potential relative to metallic lithium. That is,
oxides of titanium are more noble. Furthermore, oxides of titanium
have a lower capacity per weight. Therefore, a battery using an
oxide of titanium for the negative electrode has a problem that the
energy density is low.
For example, the electrode potential an oxide of titanium is about
1.5 V (vs. Li/Li.sup.+) relative to metallic lithium, which is
higher (i.e., more noble) in comparison to potentials of carbon
based negative electrodes. The potential of an oxide of titanium is
attributed to the redox reaction between Ti.sup.3+ and Ti.sup.4+
upon electrochemical insertion and extraction of lithium, and is
therefore electrochemically restricted. It is also a fact that
rapid charge/discharge of lithium ions can be performed stably at a
high electrode potential of about 1.5 V (vs. Li/Li.sup.+).
Conventionally, it has therefore been difficult to drop the
potential of the electrode in order to improve the energy
density.
On the other hand, considering the capacity per unit weight, the
theoretical capacity of titanium dioxide (anatase structure) is
about 165 mAh/g, and the theoretical capacity of spinel type
lithium-titanium composite oxides such as Li.sub.4Ti.sub.5O.sub.12
is about 180 mAh/g. On the other hand, the theoretical capacity of
a general graphite based electrode material is 385 mAh/g and
greater. As such, the capacity density of an oxide of titanium is
significantly lower than that of the carbon based negative
electrode material. This is due to there being only a small number
of lithium-insertion sites in the crystal structure, and lithium
tending to be stabilized in the structure, and thus, substantial
capacity being reduced.
In consideration of the above circumstances, a new electrode
material containing Ti and Nb has been studied. Such a
niobium-titanium composite oxide material is expected to have a
high charge/discharge capacity. Particularly, a composite oxide
represented by TiNb.sub.2O.sub.7 has a high theoretical capacity
exceeding 380 mAh/g. Therefore, a niobium-titanium composite oxide
is expected as a high-capacity material to replace
Li.sub.4Ti.sub.5O.sub.12, but there is a problem that a lattice
volume fluctuates during a charge-and-discharge cycle and the
contact between particles gets worse to cause deterioration in
electronic conduction network in the electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view illustrating a crystal structure of a
niobium-titanium composite oxide Nb.sub.2TiO.sub.7;
FIG. 2 is a schematic view illustrating the crystal structure of
FIG. 1 from another direction;
FIG. 3 is a plan view schematically illustrating particles to be
measured in a transmission electron microscope (TEM)
observation;
FIG. 4 is a cross-sectional view schematically illustrating an
example of a secondary battery according to an embodiment;
FIG. 5 is an enlarged cross-sectional view of a section A of the
secondary battery illustrated in FIG. 4;
FIG. 6 is a partially cut-out perspective view schematically
illustrating another example of the secondary battery according to
the embodiment;
FIG. 7 is an enlarged cross-sectional view of a section B of the
secondary battery illustrated in FIG. 6;
FIG. 8 is a perspective view schematically illustrating an example
of a battery module according to an embodiment;
FIG. 9 is an exploded perspective view schematically illustrating
an example of a battery pack according to an embodiment;
FIG. 10 is a block diagram illustrating an example of an electric
circuit of the battery pack illustrated in FIG. 9;
FIG. 11 is a cross-sectional view schematically illustrating an
example of a vehicle according to an embodiment;
FIG. 12 is a diagram schematically illustrating another example of
the vehicle according to the embodiment.
DETAILED DESCRIPTION
According to a first embodiment, an active material is provided.
The active material includes a primary particle containing a
phosphorus-containing monoclinic niobium-titanium composite oxide.
The primary particle has a concentration gradient in which a
phosphorus concentration increases from the gravity point of the
primary particle toward the surface of the primary particle.
According to a second embodiment, an electrode is provided. The
electrode includes the active material according to the first
embodiment.
According to a third embodiment, a secondary battery is provided.
The secondary battery includes the electrode according to the
second embodiment.
According to a fourth embodiment, a battery module is provided. The
battery module includes a plurality of the secondary batteries
according to the third embodiment.
According to a fifth embodiment, a battery pack is provided. The
battery pack includes the secondary battery according to the third
embodiment or the battery module according to the fourth
embodiment.
According to a sixth embodiment, a vehicle is provided. The vehicle
includes the battery pack according to the fifth embodiment.
Hereinafter, embodiments will be described with reference to the
drawings. The same reference signs are applied to common components
throughout the embodiments and overlapped explanations are thereby
omitted. Each drawing is a schematic view for encouraging
explanations of the embodiment and understanding thereof, and thus
there are some details in which a shape, a size and a ratio are
different from those in a device actually used, but they can be
appropriately design-changed considering the following explanations
and known technology.
First Embodiment
According to a first embodiment, an active material is provided.
This active material includes a primary particle containing a
phosphorus-containing monoclinic niobium-titanium composite oxide.
The primary particle has a concentration gradient in which a
phosphorus concentration increases from the gravity point of the
primary particle toward the surface of the primary particle.
Hereinafter, the reason why the active material according to the
embodiment can realize a secondary battery capable of achieving
excellent rate characteristics will be described.
First, as an example of monoclinic niobium-titanium composite
oxide, Nb.sub.2TiO.sub.7 phase will be described.
The main phase contained in the active material according to the
embodiment is a niobium-titanium composite oxide phase represented
by Nb.sub.2TiO.sub.7 as a representative composition. A composition
of the niobium-titanium composite oxide preferably has a crystal
structure having a symmetry of the space group C2/m and an atomic
coordination described in "M. Gasperin, Journal of Solid-State
Chemistry 53, pp. 144-147 (1984)" although not limited thereto.
Schematic views of the crystal structure of monoclinic
Nb.sub.2TiO.sub.7 are illustrated in FIGS. 1 and 2.
As illustrated in FIG. 1, in the crystal structure of monoclinic
Nb.sub.2TiO.sub.7, a metal ion 101 and an oxide ion 102 form a
skeleton structure section 103. At a position of the metal ion 101,
Nb ions and Ti ions are arbitrarily arranged at a ratio of
Nb:Ti=2:1. Such skeleton structures 103 are alternately arranged
three-dimensionally, thereby vacancies 104 are formed among the
skeleton structures 103. These vacancies 104 serve as hosts for
lithium ions. Lithium ions can be inserted in this crystal
structure from 0 moles up to a maximum of 5.0 moles. Therefore, the
composition when 0 to 5.0 moles of lithium ions are inserted can be
expressed as Li.sub.xNb.sub.2TiO.sub.7 (0.ltoreq.x.ltoreq.5).
In FIG. 1, regions 105 and 106 are sections having two-dimensional
channels in [100] and [010] directions. As illustrated in FIG. 2,
the crystal structure of monoclinic Nb.sub.2TiO.sub.7 has a vacancy
107 along a [001] direction. This vacancy 107 has a tunnel
structure advantageous in conduction of lithium ions and serves as
an electrically conductive path in a [001] direction connecting
region 105 and region 106. This electrically conductive path makes
it possible for the lithium ions to migrate between regions 105 and
106. Further, the niobium-titanium composite oxide has a lithium
insertion potential of about 1.5 V (vs. Li/Li.sup.+). Therefore, an
electrode including the niobium-titanium composite oxide as the
active material can realize a battery that can stably repeat rapid
charging and discharging.
When a lithium ion is inserted into the vacancy 104 in the above
crystal structure, the metal ion 101, which forms the skeleton, is
reduced to a trivalent, thereby maintaining electric neutrality of
a crystal. In the niobium-titanium composite oxide, not only a Ti
ion is reduced from tetravalent to trivalent, but also a Nb ion is
reduced from pentavalent to trivalent. Therefore, the number of
reduced valences per active material weight is large. Therefore, it
is possible to maintain electric neutrality of the crystal even
when many lithium ions are inserted. Thus, energy density is high
as compared with that of a compound such as titanium oxide
containing only a tetravalent cation.
The active material according to the embodiment includes primary
particles containing a phosphorus-containing monoclinic
niobium-titanium composite oxide. When the phosphorus concentration
gradually increases from the gravity point of the primary particle
toward the surface of the primary particle, it is possible to
improve a diffusion rate of lithium ions into the primary particle
while ensuring a practical battery capacity. The reason for this
will be described.
A phosphorus atom shows strong covalent bonding with an oxide ion
in an oxide. When lithium ions diffuse in a niobium titanium oxide,
the electron correlation between Li and O may be a barrier, but due
to the strong covalent bonding between the phosphorus atom and the
oxide ion, the electron correlation between Li and O tends to
weaken. As a result, the diffusion rate of lithium ions is
improved.
On the other hand, when the niobium-titanium composite oxide
contains a phosphorus atom, the amount of Nb that can be reduced at
the time of charge and discharge decreases, which leads to a
decrease in charge/discharge capacity. Therefore, by generating a
concentration gradient in the primary particles, the diffusion rate
of lithium ions can be improved in a portion where the phosphorus
concentration is high, and a practical charge/discharge capacity
can be achieved in a portion where the phosphorus concentration is
low (near the gravity point of the particles).
When lithium ions diffuse in the primary particle, the diffusion
distance from the vicinity of the particle surface to the inside of
the particle (near the particle center) is long and the activation
energy for diffusion is high. Therefore, it is possible to
efficiently diffuse lithium ions throughout the primary particle by
increasing the phosphorus concentration in the vicinity of the
particle surface and facilitating the diffusion of lithium ions
from the vicinity of the particle surface toward the inside of the
particle. Furthermore, since the phosphorus concentration in the
interior of the particle is low, more lithium ions can be inserted,
so that a high charge/discharge capacity can be achieved.
Regarding the concentration gradient of phosphorus, the ratio
(C2/C1) is preferably in the range of 1.05 to 100. Here, C1 is the
phosphorus concentration at the position of the gravity point of
the primary particle, C2 is the phosphorus concentration at the
position corresponding to 80% of the length defined from the
gravity point to the surface of the primary particle concentration.
The ratio (C2/C1) is more preferably in the range of 1.05 to
10.0.
The concentrations C1 and C2 can be confirmed by transmission
electron microscope (TEM) observation with energy dispersive X-ray
spectrometry (EDX) function. According to the TEM-EDX observation,
the distribution of each crystal in a material having a mixed phase
(active material) can be confirmed, and the distribution of each
element can be visualized and the element concentration can be
obtained. A specific method of TEM-EDX observation will be
described later.
The concentration C1 is, for example, in the range of 0.01 atm % to
6 atm %, preferably in the range of 0.1 atm % to 1.0 atm %. The
concentration C2 is, for example, in the range of 0.0105 atm % to
15 atm %, preferably in the range of 0.0105 atm % to 5.5 atm %.
Phosphorus present in the primary particle can be present in, for
example, a state in which phosphorus has been substituted at the
site of Nb ion in the crystal structure of the monoclinic
niobium-titanium composite oxide, and a state in which phosphorus
is precipitated at the crystal grain boundary. Alternatively, a
phosphate compound may be present at least a part of the surface of
the primary particle.
In the case where phosphorus has been substituted for the site of
Nb ion, as described above, the covalent bonding between the
phosphorus atom and the oxide ion is high, so that the diffusion
rate of lithium ions can be improved. Phosphorus present at the
crystal grain boundary of the primary particle can be present in
the form of phosphorus oxide and/or phosphate compound. When the
phosphorus oxide and/or phosphate compound is present in the
crystal grain boundary, the binding between the particles becomes
stronger and even when a crystal lattice volume varies during a
charge-and-discharge cycle, particles are hardly separated, so that
a favorable electrically conductive path can be formed.
It is preferable that a phosphate compound is present at least a
part of the surface of the primary particle because, as will be
described later, the binding property between the primary particles
at the time of formation of secondary particles can be enhanced to
form a favorable electrically conductive path. In addition, when
the phosphate compound is present on the surface of the primary
particle, the active material does not come into direct contact
with the electrolyte, and there is thus an advantage that
decomposition reaction (side reaction) of the electrolyte during
charge and discharge hardly occurs.
The type of the phosphate compound present at the crystal grain
boundary of the primary particle and the surface of the primary
particle is not particularly limited, but examples thereof include
phosphate such as lithium phosphate, potassium phosphate, sodium
phosphate, magnesium phosphate, titanium phosphate, zirconium
phosphate, manganese phosphate, iron phosphate, aluminum phosphate,
tantalum phosphate, tungsten phosphate, niobium phosphate,
molybdenum phosphate, and bismuth phosphate.
It is preferable that the phosphate compound present at least a
part of the surface of the primary particle be at least one
selected from the group consisting of phosphorus oxide, iron
phosphate, and potassium phosphate. The phosphate compound present
at least a part of the surface of the primary particle may be at
least one selected from the group consisting of iron phosphate and
potassium phosphate.
When phosphorus oxide is present on the surface of the primary
particles, a dehydrating effect of reacting a trace amount of
moisture contained in the nonaqueous electrolyte with phosphorus
oxide is obtained. As a result, it is possible to suppress the
amount of hydrogen produced during charge and discharge and
suppress the swelling of the battery in a long-term
charge-and-discharge cycle.
When iron phosphate is present on the surface of primary particles,
electron conductivity of the particle surface can be enhanced. This
is because the iron ion is reduced at the time of charge and
discharge and the electron conductivity is exhibited.
When potassium phosphate is present on the surface of primary
particles, affinity between the primary particle and the lithium
ion can be enhanced. This is because potassium is the same alkali
metal element as lithium.
The phosphorus-containing monoclinic niobium-titanium composite
oxide according to the embodiment has, for example, an average
composition represented by the general formula
Ti.sub.1-xM1Nb.sub.2-y-zM2.sub.yP.sub.zO.sub.7. In the above
general formula, 0.ltoreq.x<1, 0.ltoreq.y<1, and
0<z.ltoreq.0.5. each of the elements M1 and M2 is at least one
selected from the group consisting of V, Ta, Fe, Bi, Cr, Mo, W, B,
K, Na, Mg, Al, and Si. The element M1 and the element M2 may be the
same element or different elements from each other.
As the element M1, it is preferable to use at least one element of
Cr, Fe, and Al. These elements are trivalent elements. Therefore,
when these elements are used as the element M1, the electron
conductivity of the monoclinic niobium-titanium composite oxide can
be improved. Therefore, when these elements are used as the element
M1, the capacity and rapid charge/discharge performance of the
battery can be improved.
From the viewpoint of improving electron conductivity, it is more
preferable to use at least one element selected from the group
consisting of V, Ta, and Bi as the element M1. Since these elements
are pentavalent elements, the electron conductivity of the
monoclinic niobium-titanium composite oxide can be further
improved.
As the element M1, it is preferable to use at least one element
selected from the group consisting of B, K, Na, Mg, and Si. Atomic
weights of these elements are each smaller than an atomic weight of
Ti. Therefore, when these elements are used as the element M1, the
capacity of the battery can be increased.
As the element M2, it is preferable to use at least one element
selected from the group consisting of Mo and W. Since these
elements are hexavalent elements, the electron conductivity of the
monoclinic niobium-titanium composite oxide can be improved.
When Ta is used as the element M2, it is possible to obtain a
monoclinic niobium-titanium composite oxide having an equivalent
performance to that in the case of using Nb as the element M2. This
is considered to be because Nb and Ta have similar physical,
chemical and electrical properties.
As the elements M1 and M2, at least one element selected from the
group consisting of Mo, W, and V may be used. These elements
exhibit an effect as a sintering aid. Therefore, when these
elements are used as at least one of M1 and M2, a firing
temperature at the time of manufacturing the monoclinic
niobium-titanium composite oxide can be lowered.
A content of each of the elements M1 and M2 in the compound
represented by the general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7 can be
quantified by, for example, inductively coupled plasma (ICP)
spectroscopic analysis.
Note that the active material according to the first embodiment may
contain an oxide having a composition deviating from a
stoichiometric ratio represented by the general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7. Such an oxide
can be represented by the general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7+.delta.
(0.ltoreq.x<1, 0.ltoreq.y<1, 0<z.ltoreq.0.5,
-0.3.ltoreq..delta..ltoreq.0.3).
That is, during preparation of the phosphorus-containing monoclinic
niobium-titanium composite oxide, oxygen deficiency may occur in a
raw material or an intermediate product. In addition, inevitable
impurities contained in the raw material and impurities mixed in
the preparation may be present in the composite oxide in some
cases. Due to such unavoidable factors, a phosphorus-containing
monoclinic niobium-titanium composite oxide containing an oxide
having a composition of the stoichiometric ratio may be prepared in
some cases. The oxide having a composition deviating from the
stoichiometric ratio as above is excellent in lithium-ion insertion
stability as is that of an oxide having a composition of the
stoichiometric ratio. Therefore, even when the
phosphorus-containing monoclinic niobium-titanium composite oxide
contains the oxide having a composition deviating from such a
stoichiometric ratio, the influence on the lithium-ion insertion
capacity is small.
In addition, the primary particles according to the embodiment may
contain a monoclinic niobium-titanium composite oxide phase not
containing phosphorus. The monoclinic niobium-titanium composite
oxide phase not containing phosphorus is represented by, for
example, a general formula
Ti.sub.1-xM3.sub.xNb.sub.2-yM4.sub.yO.sub.7. In the above general
formula, 0.ltoreq.x<1, 0.ltoreq.y<1, each of the elements M3
and M4 is at least one selected from the group consisting of V, Ta,
Fe, Bi, Cr, Mo, W, B, K, Na, Mg, Al, and Si, and the elements M3
and M4 may be the same element or different elements from each
other. The primary particles according to the embodiment may
contain a heterogeneous phase different in Nb/Ti ratio from this
general formula. Examples of such different phases are rutile type
TiO.sub.2, Nb.sub.24TiO.sub.62, Nb.sub.14TiO.sub.37, and
Nb.sub.10Ti.sub.2O.sub.29.
The lattice volume of the niobium-titanium composite oxide
fluctuates due to charge and discharge. Therefore, when the
niobium-titanium composite oxide is present as primary particles in
the electrode, the state of contact between a binder and a
conductive agent constituting the active material-containing layer
and the active material particle tends to change. Therefore,
peeling of the active material-containing layer from the current
collector and peeling of the active material particle from the
conductive agent occur during repeated charge and discharge, so
that the electrically conductive path in the electrode is easily
shredded.
Therefore, the active material according to the embodiment
preferably contains the secondary particles formed by granulation
of a plurality of primary particles via the phosphate compound. In
other words, a phosphate compound is present between a plurality of
primary particles contained in such a secondary particle. This
makes it possible to suppress shredding of the electrically
conductive path due to a change in lattice volume during charge and
discharge. The phosphate compound not only tightly binds the
primary particles to each other but also makes it easy for the
lithium ions to move. Therefore, the secondary particle formed by
binding the plurality of primary particles with the phosphate
compound can not only make the electrically conductive path
resistant to shredding, but also make the movement of the lithium
ions smooth by the phosphate compound interposed between the
particles. That is, the lithium ion can rapidly move between the
primary particles. Therefore, when the active material according to
the embodiment contains the secondary particle formed of a
plurality of primary particles, and the phosphate compound is
present between the plurality of primary particles, it is possible
to realize the secondary battery having excellent rate
characteristics and to suppress deterioration in the electrode when
charge and discharge are repeated. That is, in this case, a
secondary battery having excellent life characteristics can be
realized. It is more preferable that the phosphate compound present
between the plurality of primary particles contain at least one
selected from the group consisting of iron phosphate and potassium
phosphate, because the lithium ion conductivity between the
particles can be enhanced.
Next, the form, particle diameter and specific surface area of the
active material according to the embodiment will be described.
<Form>
The form of the active material according to the embodiment is, for
example, the form of the secondary particles in which the surfaces
of the primary particles is the phosphate compound, and the
secondary particles is formed by binding the primary particles via
the phosphate compound, but the active material does not need to be
formed of only the secondary particles. The active material may be
a mixture of the primary particles and the secondary particles.
A particle of the niobium-titanium composite oxide may have a
carbon-containing layer on each of the primary particle surface and
the secondary particle surface. The active material may contain a
secondary particle granulated by the carbon-containing layer
adhering to the primary particle, on the surface of which the
phosphate compound is present. Such secondary particle can exhibit
excellent conductivity because carbon is present between the
primary particles. In such an embodiment containing the secondary
particles, the secondary particles tightly bonded via the phosphate
compound and the secondary particles bonded via the
carbon-containing layer are mixed, whereby it is possible to
enhance the life performance and show lower resistance.
The ratio of the weight of the niobium-titanium composite oxide
present in the form of the secondary particles with respect to the
weight of the active material is, for example, in the range of 5.0%
by weight to 99.0% by weight.
<Particle Size>
An average particle size of the active material particles, which
are the primary particles or the secondary particles of the
niobium-titanium composite oxide, is not particularly limited. An
average particle size of the active material particle is, for
example, in the range of 0.1 .mu.m to 50 .mu.m. The average
particle size can be varied in accordance with required battery
characteristics. For example, it is preferable to set the average
particle size to 1.0 .mu.m or less in order to enhance rapid
charge/discharge performance. In this manner, it is possible to
reduce a diffusion distance between lithium ions in the crystal, so
that the rapid charge/discharge performance can be enhanced. The
average particle size can be obtained by laser diffraction, for
example.
<BET Specific Surface Area>
The BET (Brunauer, Emmett, Teller) specific surface area of the
active material according to the embodiment is not particularly
limited. However, the BET specific surface area is preferably 5
m.sup.2/g or more and less than 200 m.sup.2/g.
If the specific surface area is 5 m.sup.2/g or more, a contact area
with the electrolyte can be secured, favorable discharge rate
characteristics can be easily obtained, and the charging time can
be shortened. If the specific surface area is less than 200
m.sup.2/g, on the other hand, reactivity with the electrolyte does
not become too high so that the life performance can be improved.
Further, coating properties of a slurry used in the production of
an electrode described below and including the active material can
be made favorable.
Here, for the measurement of the specific surface area, a method is
used by which molecules, in which an occupied area in adsorption is
known, are adsorbed onto the surface of powder particles at a
temperature of liquid nitrogen and the specific surface area of the
sample is determined from the amount of adsorbed molecules. The
most commonly used is the BET method based on low-temperature and
low-humidity physical adsorption of an inert gas, which is the most
famous theory as a method of calculating the specific surface area
by extending the Langmuir theory, which is monomolecular layer
adsorption theory to multi-molecular layer adsorption. The specific
surface area determined by the above method is referred to as a
"BET specific surface area".
<TEM-EDX Observation>
Next, a method of observing a transmission electron microscope
(TEM-EDX) with an energy dispersive X-ray spectroscopic function
will be described. As described above, according to the TEM-EDX
observation, a distribution of each crystal in a material having a
mixed phase (active material) can be confirmed. It is also possible
to visualize the distribution of the element and to determine a
concentration of the element.
In the case of conducting TEM-EDX observation on the active
material contained in the electrode, for example, the observation
can be performed as follows.
First, in order to grasp the crystal state of the active material,
lithium ions are fully released from the active material. For
example, when the active material is used in the negative
electrode, the battery is brought into a fully discharged state.
The battery can be brought into the discharged state by, for
example, repeating the discharge of the battery at 0.1 C current at
25.degree. C. until a rated end voltage or a battery voltage
reaches 1.0 V a plurality of times, so that the current value at
the time of discharge becomes 1/100 or less of the rated capacity.
There may be lithium ions remaining even in the discharged
state.
Next, the battery is dissembled in a glove box filled with argon,
and the electrode is taken out and washed with an appropriate
solvent. As an appropriate solvent, for example, ethyl methyl
carbonate can be used. When the cleaning of the electrode is
insufficient, an impurity phase such as lithium carbonate and
lithium fluoride may be mixed due to the influence of lithium ions
remaining in the electrode. In that case, an airtight container
capable of performing measurement atmosphere in an inert gas may be
used. At this time, peaks derived from metal foil which is the
current collector, the conductive agent, the binder, and the like
are measured in advance by using EDX and grasped. Naturally, when
you have been able to grasp these in advance, this operation can be
omitted. When the peak of the current collector and the peak of the
active material overlap, it is desirable to perform measurement by
peeling the active material-containing layer off the current
collector. This is for separating the overlapping peaks at the time
of quantitatively measuring peak intensity. The active
material-containing layer may be physically peeled. The active
material-containing layer tends to be peeled when an ultrasonic
wave is applied to the active material-containing layer in an
appropriate solvent. When ultrasonic treatment is performed to peel
the active material-containing layer off the current collector, an
electrode body powder (including the active material, the
conductive agent, and the binder) can be recovered by volatilizing
the solvent.
In the TEM-EDX measurement, it is desirable to embed a target
sample powder in resin or the like and observe the particle cross
section by scraping the inside of a specimen by mechanical
polishing and ion milling. In addition, similar treatment can be
performed even when the target sample is an electrode body. It is
also possible to embed the electrode body in the state of being the
electrode body as it is and observe a desired portion, or it is
also possible to separate the current collector (metal foil) from
the electrode body and observe the electrode powder mixed with the
conductive agent and the binder. In this manner, it is possible to
see how the niobium-titanium composite oxide, the phosphorus oxide
and/or phosphate compound are distributed in the primary particle,
and it is further possible to see the composition in the particle.
For example, when the surface of the primary particle is a
phosphate compound, it is also possible to observe a boundary
portion between the phosphate compound and the niobium-titanium
composite oxide phase present inside the phosphate compound. In
addition, by observing the contact portion between the primary
particles at a relatively low magnification, it can be confirmed
whether or not the secondary particle is contained in the powder to
be measured.
A specific example will be described with reference to FIG. 3. FIG.
3 is a plan view schematically showing a cross section of an object
to be measured. First, the gravity point of a primary particle 50
to be measured is regarded as the center of the particle
(measurement point A). Next, five points X are set at equal
intervals on a straight line connecting the particle gravity point
and an arbitrary point on the particle surface. In a region
orthogonal to each point X, a point corresponding to 80% of the
distance from the gravity point toward an outer shell (surface) to
the particle surface is regarded as a measurement point B. For each
measurement point B, an electron beam diffraction pattern is
observed. At this time, by examining a multi-wave interference
image, phosphorus oxide or phosphate compound can be searched
separately from the monoclinic niobium-titanium composite oxide
phase separately. For example, by simulating the electron beam
diffraction pattern in advance, it is possible to easily
distinguish the monoclinic niobium-titanium composite oxide phase,
phosphorus oxide, the phosphate compound and other phases. Next, an
amount of phosphorus in the particle cross section is mapped using
the EDX. Then, a concentration (atm %) at the central portion
(measurement point A) of the particle and a concentration (atm %)
at the measurement point B at each of the five places are measured.
For the measurement point B, an average value of the concentrations
at the five places is calculated. This measurement is performed on
ten randomly selected particles.
A concentration C1 is an average concentration (atm %) obtained by
averaging the phosphorus concentrations at the measurement point A,
measured for the ten randomly selected particles. A concentration
C2 is an average concentration (atm %) obtained by further
averaging the average values of phosphorus concentrations at five
measurement points B, measured the for ten randomly selected
particles. When the concentration C2 is higher than the
concentration C1, the active material particles to be measured can
be regarded as having a concentration gradient continuously
increasing from the gravity point toward the surface.
The crystal structure of the phosphate compound contained in the
active material and the niobium-titanium composite oxide of the
primary particle portion (core particle portion) can be confirmed
by powder X-ray diffraction measurement and transmission electron
microscope (TEM) observation, for example.
<Measurement of Powder X-Ray Diffraction of Active
Material>
The powder X-ray diffraction measurement of the active material can
be performed, for example, as follows.
First, the target sample is ground until an average particle size
reaches about 5 .mu.m. A holder part, which has a depth of 0.2 mm
and is formed on a glass sample plate, is filled with the ground
sample. At this time, care should be taken to fill the holder part
sufficiently with the sample. In addition, Precaution should be
taken to perform the filling with the amount of the sample neither
being excessive nor insufficient such that cracks, voids, and the
like do not occur. Next, another glass plate is pushed from the
outside to flatten a surface of the sample filling the holder part.
Precaution should be taken not to cause a recess or a protrusion
from a reference plane of the holder due to an excessive or
insufficient amount of filling.
Next, the glass plate filled with the sample is set in a powder
X-ray diffractometer, and a diffraction pattern (X-Ray diffraction
pattern (XRD pattern)) is obtained using Cu-K.alpha. rays.
Incidentally, there is a case where the orientation of the sample
increases depending on a particle shape of the sample. In the case
where there is high degree of orientation in the sample, there is
the possibility of deviation of the peak or variation in an
intensity ratio, depending on the filling state of the sample. The
sample whose orientation is remarkably high in this manner is
measured using a glass capillary. Specifically, a sample is
inserted into a capillary, and this capillary is placed on a rotary
sample stage and measured. It is possible to alleviate the
orientation with the above-described measuring method. It is
preferable to use a capillary formed of Lindeman glass having a
diameter of 1 mm to 6 mm.phi. as the glass capillary.
When the powder X-ray diffraction measurement is performed on the
active material contained in the electrode, the measurement is
performed, for example, as follows.
First, in order to grasp the crystal state of the active material,
lithium ions are fully released from the active material. As this
operation, for example, the battery can be brought into the
discharged state by the method described in the section of the
TEM-EDX observation.
Next, the battery is dissembled in a glove box filled with argon,
and the electrode is taken out and washed with an appropriate
solvent. As an appropriate solvent, for example, ethyl methyl
carbonate can be used. When the cleaning of the electrode is
insufficient, an impurity phase such as lithium carbonate and
lithium fluoride may be mixed due to the influence of lithium ions
remaining in the electrode. In that case, an airtight container
capable of performing measurement atmosphere in an inert gas may be
used. The washed electrode is cut so as to have an area
approximately equal to an area of the holder of the powder X-ray
diffraction apparatus, to obtain a measurement sample. The sample
is directly attached to a glass holder for measurement.
At this time, peaks derived from metal foil which is the current
collector, the conductive agent, the binder, and the like are
measured in advance by using XRD and grasped. Naturally, when you
have been able to grasp these in advance, this operation can be
omitted. When the peak of the current collector and the peak of the
active material overlap, it is desirable to perform measurement by
peeling the active material-containing layer off the current
collector. This is for separating the overlapping peaks at the time
of quantitatively measuring peak intensity. The active
material-containing layer may be physically peeled. The active
material-containing layer tends to be peeled when an ultrasonic
wave is applied to the active material-containing layer in an
appropriate solvent. When ultrasonic treatment is performed to peel
the active material-containing layer off the current collector, an
electrode body powder (including the active material, the
conductive agent, and the binder) can be recovered by volatilizing
the solvent. Powder X-ray diffraction measurement of the active
material can be performed by filling, for example, a Lindemann
glass capillary or the like with the collected electrode body
powder and measuring the electrode body powder. Note that the
electrode body powder recovered by the ultrasonic treatment can
also be subjected to various analyses other than the powder X-ray
diffraction measurement.
In the obtained diffraction peak, a mixed chart of a peak
attributed to the niobium-titanium composite oxide phase having the
maximum peak intensity and a peak attributed to the phosphate
compound is observed.
<ICP Emission Spectroscopy>
The composition of the active material can be analyzed using, for
example, inductively coupled plasma (ICP) emission spectroscopy. At
this time, an abundance ratio (molar ratio) of each element depends
on sensitivity of an analyzer to be used. Hence the measured molar
ratio may deviate from an actual molar ratio by an error of the
measuring device. However, even when the numerical value deviates
from the error range of the analyzer, the performance of the active
material according to the embodiment can be exhibited
sufficiently.
For measuring the composition of the active material incorporated
in the battery by the ICP emission spectroscopy, specifically, the
following procedure is performed.
First, the electrode containing the active material to be measured
is taken out from the secondary battery by the procedure described
in the section of the powder X-ray diffraction measurement, and
then washed. From the washed electrode, the portion containing the
electrode active material such as the active material-containing
layer is peeled. The portion containing the electrode active
material can be peeled by, for example, irradiating the portion
with ultrasonic waves. As a specific example, for example, by
placing the electrode in ethyl methyl carbonate placed in a glass
beaker and vibrating the electrode in an ultrasonic washer, it is
possible to peel the active material-containing layer containing
the electrode active material off the electrode current
collector.
Next, the peeled portion is heated in an atmosphere for a short
time (e.g., at 500.degree. C. for about 1 hour) to burn off
unnecessary components such as the binder component and carbon. By
dissolving this residue with acid, a liquid sample containing an
active material can be prepared. At this time, hydrochloric acid,
nitric acid, sulfuric acid, hydrogen fluoride, and the like can be
used as the acid. The composition in the active material can be
seen by subjecting this liquid sample to ICP analysis.
<Manufacturing Method>
The active material according to the embodiment can be produced by
a first synthesis method described below.
(First Synthesis Method)
First, in addition to various salts such as niobium oxide, titanium
oxide, and oxides or carbonates of additional elements (M1 and M2
below), P.sub.2O.sub.5 as a phosphorus source is prepared. At this
time, for example, the phosphorus source is added so that
phosphorus is in excess of 10 mol % to 500 mol %, more than a molar
amount represented by the general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7, which is a
target composition. Next, after heating at 350.degree. C. for 2
hours, firing is performed at 800.degree. C. for 12 hours and at
1000.degree. C. for 2 hours.
Thereafter, by rapidly quenching to room temperature or lower, the
phosphorus-containing niobium-titanium composite oxide particle
having a concentration gradient of phosphorus can be obtained. The
rapidly quenching to the room temperature or lower can be
performed, for example, by putting the powder after firing into
liquid nitrogen. The phosphorus oxide is decomposed by heating at
350.degree. C., the phosphorus oxide is uniformly dispersed inside
the raw material particle by heating at 800.degree. C., and by
heating at 1000.degree. C., it is possible to obtain the
phosphorus-containing niobium-titanium composite oxide having the
concentration gradient of phosphorus and the target composition.
Excessive phosphorus present inside the particle can precipitates
toward the crystal grain boundary portion (primary particle
surface) between the primary particles during firing. By rapidly
quenching in this process, it is possible to obtain the primary
particle containing the phosphorus-containing niobium-titanium
composite oxide having the concentration gradient of
phosphorus.
According to the first synthesis method, a particle in which the
surface of primary particle is phosphorus oxide can also be
produced. Further, according to the first synthesis method, a
phosphate compound is present between a plurality of primary
particles, and a secondary particle formed by granulation of these
primary particles can also be produced.
Furthermore, according to the second synthesis method described
below, it is possible to produce a particle in which the primary
particle surface of the phosphorus-containing monoclinic
niobium-titanium composite oxide is a phosphate compound except for
phosphorus oxide. As described above, examples of the phosphate
compound include lithium phosphate, potassium phosphate, sodium
phosphate, magnesium phosphate, titanium phosphate, zirconium
phosphate, manganese phosphate, iron phosphate, aluminum phosphate,
tantalum phosphate, tungsten phosphate, niobium phosphate,
molybdenum phosphate, bismuth phosphate, and the like.
(Second Synthesis Method)
First, in addition to various salts such as niobium oxide, titanium
oxide, and oxides or carbonates of additional elements (M1 and M2
below), P.sub.2O.sub.5 as a phosphorus source is prepared. At this
time, for example, the phosphorus source is added so that
phosphorus is in excess of 10 mol % to 500 mol %, more than a molar
amount represented by the general formula
Ti.sub.1-xM1.sub.xNb.sub.2-y-zM2.sub.yP.sub.zO.sub.7, which is a
target composition. Next, after heating at 350.degree. C. for 2
hours, cooling is performed to room temperature.
To the powder obtained after cooling, for example, oxides or
carbonates of lithium, potassium, sodium, magnesium, titanium,
zirconium, manganese, iron, aluminum, tantalum, tungsten, niobium,
molybdenum and bismuth are added in a predetermined number of
moles. For example, in the case of producing an active material
particle containing the iron phosphate and/or potassium phosphate
on the surface of the primary particle, a predetermined number of
moles of iron oxide and/or potassium carbonate are added.
Then, firing is performed at 800.degree. C. for 12 hours and at
1000.degree. C. for 2 hours. Thereafter, by rapidly quenching to
room temperature or lower, the niobium-titanium composite oxide
particle having a concentration gradient of phosphorus can be
obtained. The rapidly quenching to the room temperature or lower
can be performed, for example, by putting the powder after firing
into liquid nitrogen. The phosphorus oxide is decomposed by heating
at 350.degree. C., the phosphorus oxide is uniformly dispersed
inside the raw material particle by heating at 800.degree. C., and
by heating at 1000.degree. C., it is possible to obtain the primary
particle of the phosphorus-containing niobium-titanium composite
oxide having the concentration gradient of phosphorus and the
target composition. When excessive phosphorus present inside the
particle precipitates toward the crystal grain boundary part
(primary particle surface) between the primary particles during
firing, iron and potassium react with phosphorus to form iron
phosphate and/or potassium phosphate on the primary particle
surface. On the other hand, by rapidly quenching in this process,
it is possible to obtain the primary particle containing the
phosphorus-containing niobium-titanium composite oxide having the
concentration gradient of phosphorus.
Further, according to the second synthesis method, a phosphate
compound is present between a plurality of primary particles, and a
secondary particle formed by granulation of these primary particles
can also be produced.
According to a first embodiment, an active material is provided.
The active material includes a primary particle containing a
phosphorus-containing monoclinic niobium-titanium composite oxide.
The primary particle has a concentration gradient in which a
phosphorus concentration increases from the gravity point of the
primary particle toward the surface of the primary particle.
According to the active material, a secondary battery capable of
achieving excellent rate characteristics can be realized.
Second Embodiment
According to the second embodiment, an electrode is provided.
The electrode according to the second embodiment includes the
active material according to the first embodiment. This electrode
may be a battery electrode containing the active material according
to the first embodiment as an active material for a battery. The
electrode as a battery electrode may be, for example, a negative
electrode containing the active material according to the first
embodiment as a negative electrode active material.
The electrode according to the second embodiment may include a
current collector and an active material-containing layer. The
active material-containing layer may be formed on both of reverse
surfaces or one surface of the current collector. The active
material-containing layer may contain the active material, and
optionally an electro-conductive agent and a binder.
The active material-containing layer may singly include the active
material according to the first embodiment or include two or more
kinds of the active material according to the first embodiment.
Furthermore, a mixture where one kind or two or more kinds of the
active material according to the first embodiment is further mixed
with one kind or two or more kinds of another active material may
also be included.
For example, in a case where the active material according to the
first embodiment is included as the negative electrode active
material, examples of other active materials include lithium
titanate having a ramsdellite structure (e.g.,
Li.sub.2+yTi.sub.3O.sub.7, 0<y.ltoreq.3), lithium titanate
having a spinel structure (e.g., Li.sub.4+xTi.sub.5O.sub.12,
0<x.ltoreq.3), monoclinic titanium dioxide (TiO.sub.2), anatase
type titanium dioxide, rutile type titanium dioxide, a hollandite
type titanium composite oxide, and an orthorhombic
titanium-containing composite oxide.
Examples of the orthorhombic titanium-containing composite oxide
include a compound represented by
Li.sub.2+aM(I).sub.2-bTi.sub.6-cM(II).sub.dO.sub.14+.sigma.. Here,
M(I) is at least one selected from the group consisting of Sr, Ba,
Ca, Mg, Na, Cs, Rb and K. M(II) is at least one selected from the
group consisting of Zr, Sn, V, Nb, Ta, Mo, W, Y, Fe, Co, Cr, Mn, Ni
and Al. The respective subscripts in the composition formula are
specified as follows: 0.ltoreq.a.ltoreq.6, 0.ltoreq.b<2,
0.ltoreq.c<6, 0.ltoreq.d<6, and
-0.5.ltoreq..sigma..ltoreq.0.5. Specific examples of the
orthorhombic titanium-containing composite oxide include
Li.sub.2+aNa.sub.2Ti.sub.6O.sub.14 (0.ltoreq.a.ltoreq.6).
The electro-conductive agent is added to improve current collection
performance and to suppress the contact resistance between the
active material and the current collector. Examples of the
electro-conductive agent include carbonaceous substances such as
vapor grown carbon fiber (VGCF), carbon blacks such as acetylene
black, and graphite. One of these may be used as the
electro-conductive agent, or two or more may be used in combination
as the electro-conductive agent. Alternatively, instead of using an
electro-conductive agent, a carbon coating or an electro-conductive
inorganic material coating may be applied to the surface of the
active material particle.
The binder is added to fill gaps among the dispersed active
material and also to bind the active material with the current
collector. Examples of the binder include polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVdF), fluorine rubber,
styrene-butadiene rubber, polyacrylate compounds, imide compounds,
carboxymethyl cellulose (CMC), and salts of CMC. One of these may
be used as the binder, or two or more may be used in combination as
the binder.
The blending proportion of active material, electro-conductive
agent and binder in the active material-containing layer may be
appropriately changed according to the use of the electrode. For
example, in the case of using the electrode as a negative electrode
of a secondary battery, the active material (negative electrode
active material), electro-conductive agent and binder in the active
material-containing layer are preferably blended in proportions of
68% by mass to 96% by mass, 2% by mass to 30% by mass, and 2% by
mass to 30% by mass, respectively. When the amount of
electro-conductive agent is 2% by mass or more, the current
collection performance of the active material-containing layer can
be improved. When the amount of binder is 2% by mass or more,
binding between the active material-containing layer and current
collector is sufficient, and excellent cycling performances can be
expected. On the other hand, an amount of each of the
electro-conductive agent and binder is preferably 30% by mass or
less, in view of increasing the capacity.
There may be used for the current collector, a material which is
electrochemically stable at the potential (vs. Li/Li.sup.+) at
which lithium (Li) is inserted into and extracted from active
material. For example in the case where the active material is used
as a negative electrode active material, the current collector is
preferably made of copper, nickel, stainless steel, aluminum, or an
aluminum alloy including one or more elements selected from the
group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. The thickness
of the current collector is preferably from 5 .mu.m to 20 .mu.m.
The current collector having such a thickness can maintain balance
between the strength and weight reduction of the electrode.
The current collector may include a portion where the active
material-containing layer is not formed on a surface of the current
collector. This portion may serve as an electrode tab.
The electrode may be produced by the following method, for example.
First, active material, electro-conductive agent, and binder are
suspended in a solvent to prepare a slurry. The slurry is applied
onto one surface or both of reverse surfaces of a current
collector. Next, the applied slurry is dried to form a layered
stack of active material-containing layer and current collector.
Then, the layered stack is subjected to pressing. The electrode can
be produced in this manner.
Alternatively, the electrode may also be produced by the following
method. First, active material, electro-conductive agent, and
binder are mixed to obtain a mixture. Next, the mixture is formed
into pellets. Then the electrode can be obtained by arranging the
pellets on the current collector.
The electrode according to the second embodiment includes the
active material according to the first embodiment. Therefore, the
electrode can realize a secondary battery capable of achieving
excellent rate characteristics.
Third Embodiment
According to a third embodiment, there is provided a secondary
battery including a negative electrode, a positive electrode, and
an electrolyte. The secondary battery includes the electrode
according to the second embodiment as the negative electrode. That
is, the secondary battery according to the third embodiment
includes, as the negative electrode, the electrode containing the
active material according to the first embodiment as a battery
active material.
The secondary battery according to the third embodiment may further
include a separator provided between the positive electrode and the
negative electrode. The negative electrode, the positive electrode,
and the separator can structure an electrode group. The electrolyte
may be held in the electrode group.
The secondary battery according to the third embodiment may further
include a container member that houses the electrode group and the
electrolyte.
The secondary battery according to the third embodiment may further
include a negative electrode terminal electrically connected to the
negative electrode and a positive electrode terminal electrically
connected to the positive electrode.
The secondary battery according to the third embodiment may be, for
example, a lithium ion secondary battery. The secondary battery
also includes nonaqueous electrolyte secondary batteries containing
nonaqueous electrolyte(s).
Hereinafter, the negative electrode, the positive electrode, the
electrolyte, the separator, the container member, the negative
electrode terminal, and the positive electrode terminal will be
described in detail.
(1) Negative Electrode
The negative electrode may include a negative electrode current
collector and a negative electrode active material-containing
layer. The negative electrode current collector and the negative
electrode active material-containing layer may be respectively a
current collector and an active material-containing layer that may
be included in the electrode according to the second embodiment.
The negative electrode active material-containing layer contains
the active material according to the first embodiment as a negative
electrode active material.
Of the details of the negative electrode, parts overlapping with
the details described in the second embodiment are omitted.
The density of the negative electrode active material-containing
layer (excluding the current collector) is preferably from 1.8
g/cm.sup.3 to 3.5 g/cm.sup.3. The negative electrode, in which the
density of the negative electrode active material-containing layer
is within this range, is excellent in energy density and ability to
hold the electrolyte. The density of the negative electrode active
material-containing layer is more preferably from 2.5 g/cm.sup.3 to
2.9 g/cm.sup.3.
The negative electrode may be produced by a method similar to that
for the electrode according to the second embodiment, for
example.
(2) Positive Electrode
The positive electrode may include a positive electrode current
collector and a positive electrode active material-containing
layer. The positive electrode active material-containing layer may
be formed on one surface or both of reverse surfaces of the
positive electrode current collector. The positive electrode active
material-containing layer may include a positive electrode active
material, and optionally an electro-conductive agent and a
binder.
As the positive electrode active material, for example, an oxide or
a sulfide may be used. The positive electrode may singly include
one kind of compound as the positive electrode active material, or
alternatively, include two or more kinds of compounds in
combination. Examples of the oxide and sulfide include compounds
capable of having Li and Li ions be inserted and extracted.
Examples of such compounds include manganese dioxides (MnO.sub.2),
iron oxides, copper oxides, nickel oxides, lithium manganese
composite oxides (e.g., Li.sub.xMn.sub.2O.sub.4 or
Li.sub.xMnO.sub.2; 0<x.ltoreq.1), lithium nickel composite
oxides (e.g., Li.sub.xNiO.sub.2; 0<x.ltoreq.1), lithium cobalt
composite oxides (e.g., Li.sub.xCoO.sub.2; 0<x.ltoreq.1),
lithium nickel cobalt composite oxides (e.g.,
Li.sub.xNi.sub.1-yCo.sub.yO.sub.2; 0<x.ltoreq.1, 0<y<1),
lithium manganese cobalt composite oxides (e.g.,
Li.sub.xMn.sub.yCo.sub.1-yO.sub.2; 0<x.ltoreq.1, 0<y<1),
lithium manganese nickel composite oxides having a spinel structure
(e.g., Li.sub.xMn.sub.2-yO.sub.4; 0<x.ltoreq.1, 0<y<2),
lithium phosphates having an olivine structure (e.g.,
Li.sub.xFePO.sub.4; 0<x.ltoreq.1,
Li.sub.xFe.sub.1-yMn.sub.yPO.sub.4; 0<x.ltoreq.1, 0<y<1,
and Li.sub.xCoPO.sub.4; 0<x.ltoreq.1), iron sulfates
[Fe.sub.2(SO.sub.4).sub.3], vanadium oxides (e.g., V.sub.2O.sub.5),
and lithium nickel cobalt manganese composite oxides
(Li.sub.xNi.sub.1-y-zCo.sub.yMn.sub.zO.sub.2; 0<x.ltoreq.1,
0<y<1, 0<z<1, y+z<1).
Among the above, examples of compounds more preferable as the
positive electrode active material include lithium manganese
composite oxides having a spinel structure (e.g.,
Li.sub.xMn.sub.2O.sub.4; 0<x.ltoreq.1), lithium nickel composite
oxides (e.g., Li.sub.xNiO.sub.2; 0<x.ltoreq.1), lithium cobalt
composite oxides (e.g., Li.sub.xCoO.sub.2; 0<x<1), lithium
nickel cobalt composite oxides (e.g.,
Li.sub.xNi.sub.1-yCo.sub.yO.sub.2; 0<x.ltoreq.1, 0<y<1),
lithium manganese nickel composite oxides having a spinel structure
(e.g., Li.sub.xMn.sub.2-yNi.sub.yO.sub.4; 0<x.ltoreq.1,
0<y<2), lithium manganese cobalt composite oxides (e.g.,
Li.sub.xMn.sub.yCo.sub.1-yO.sub.2; 0<x.ltoreq.1, 0<y<1),
lithium iron phosphates (e.g., Li.sub.xFePO.sub.4;
0<x.ltoreq.1), and lithium nickel cobalt manganese composite
oxides (Li.sub.xNi.sub.1-y-zCo.sub.yMn.sub.zO.sub.2;
0<x.ltoreq.1, 0<y<1, 0<z<1, y+z<1). The positive
electrode potential can be made high by using these positive
electrode active materials.
When a room temperature molten salt is used as the electrolyte of
the battery, it is preferable to use a positive electrode active
material including lithium iron phosphate, Li.sub.xVPO.sub.4F
(0.ltoreq.x.ltoreq.1), lithium manganese composite oxide, lithium
nickel composite oxide, lithium nickel cobalt composite oxide, or a
mixture thereof. Since these compounds have low reactivity with
room temperature molten salts, cycle life can be improved. Details
regarding the room temperature molten salt are described later.
The primary particle size of the positive electrode active material
is preferably from 100 nm to 1 .mu.m. The positive electrode active
material having a primary particle size of 100 nm or more is easy
to handle during industrial production. In the positive electrode
active material having a primary particle size of 1 .mu.m or less,
diffusion of lithium ions within solid can proceed smoothly.
The specific surface area of the positive electrode active material
is preferably from 0.1 m.sup.2/g to 10 m.sup.2/g. The positive
electrode active material having a specific surface area of 0.1
m.sup.2/g or more can secure sufficient sites for inserting and
extracting Li ions. The positive electrode active material having a
specific surface area of 10 m.sup.2/g or less is easy to handle
during industrial production, and can secure a good charge and
discharge cycle performance.
The binder is added to fill gaps among the dispersed positive
electrode active material and also to bind the positive electrode
active material with the positive electrode current collector.
Examples of the binder include polytetrafluoroethylene (PTFE),
polyvinylidene fluoride (PVdF), fluorine rubber, polyacrylate
compounds, imide compounds, carboxymethyl cellulose (CMC), and
salts of CMC. One of these may be used as the binder, or two or
more may be used in combination as the binder.
The electro-conductive agent is added to improve current collection
performance and to suppress the contact resistance between the
positive electrode active material and the positive electrode
current collector. Examples of the electro-conductive agent include
carbonaceous substances such as vapor grown carbon fiber (VGCF),
carbon black such as acetylene black, and graphite. One of these
may be used as the electro-conductive agent, or two or more may be
used in combination as the electro-conductive agent. The
electro-conductive agent may be omitted.
In the positive electrode active material-containing layer, the
positive electrode active material and binder are preferably
blended in proportions of 80% by mass to 98% by mass, and 2% by
mass to 20% by mass, respectively.
When the amount of the binder is 2% by mass or more, sufficient
electrode strength can be achieved. The binder may serve as an
electrical insulator. Thus, when the amount of the binder is 20% by
mass or less, the amount of insulator in the electrode is reduced,
and thereby the internal resistance can be decreased.
When an electro-conductive agent is added, the positive electrode
active material, binder, and electro-conductive agent are
preferably blended in proportions of 77% by mass to 95% by mass, 2%
by mass to 20% by mass, and 3% by mass to 15% by mass,
respectively.
When the amount of the electro-conductive agent is 3% by mass or
more, the above-described effects can be expressed. By setting the
amount of the electro-conductive agent to 15% by mass or less, the
proportion of electro-conductive agent that contacts the
electrolyte can be made low. When this proportion is low, the
decomposition of an electrolyte can be reduced during storage under
high temperatures.
The positive electrode current collector is preferably an aluminum
foil, or an aluminum alloy foil containing one or more elements
selected from the group consisting of Mg, Ti, Zn, Ni, Cr, Mn, Fe,
Cu, and Si.
The thickness of the aluminum foil or aluminum alloy foil, is
preferably from 5 .mu.m to 20 .mu.m, and more preferably 15 .mu.m
or less. The purity of the aluminum foil is preferably 99% by mass
or more. The amount of transition metal such as iron, copper,
nickel, or chromium contained in the aluminum foil or aluminum
alloy foil is preferably 1% by mass or less.
The positive electrode current collector may include a portion
where a positive electrode active material-containing layer is not
formed on a surface of the positive electrode current collector.
This portion may serve as a positive electrode tab.
The positive electrode may be produced by a method similar to that
for the electrode according to the second embodiment, for example,
using a positive electrode active material.
(3) Electrolyte
As the electrolyte, for example, a liquid nonaqueous electrolyte or
gel nonaqueous electrolyte may be used. The liquid nonaqueous
electrolyte is prepared by dissolving an electrolyte salt as solute
in an organic solvent. The concentration of electrolyte salt is
preferably from 0.5 mol/L to 2.5 mol/L.
Examples of the electrolyte salt include lithium salts such as
lithium perchlorate (LiClO.sub.4), lithium hexafluorophosphate
(LiPF.sub.6), lithium tetrafluoroborate (LiBF.sub.4), lithium
hexafluoroarsenate (LiAsF.sub.6), lithium trifluoromethanesulfonate
(LiCF.sub.3SO.sub.3), and lithium bistrifluoromethylsulfonylimide
[LiN(CF.sub.3SO.sub.2).sub.2], and mixtures thereof. The
electrolyte salt is preferably resistant to oxidation even at a
high potential, and most preferably LiPF.sub.6.
Examples of the organic solvent include cyclic carbonates such as
propylene carbonate (PC), ethylene carbonate (EC), or vinylene
carbonate (VC); linear carbonates such as diethyl carbonate (DEC),
dimethyl carbonate (DMC), or methyl ethyl carbonate (MEC); cyclic
ethers such as tetrahydrofuran (THF), 2-methyl tetrahydrofuran
(2-MeTHF), or dioxolane (DOX); linear ethers such as dimethoxy
ethane (DME) or diethoxy ethane (DEE); .gamma.-butyrolactone (GBL),
acetonitrile (AN), and sulfolane (SL). These organic solvents may
be used singularly or as a mixed solvent.
The gel nonaqueous electrolyte is prepared by obtaining a composite
of a liquid nonaqueous electrolyte and a polymeric material.
Examples of the polymeric material include polyvinylidene fluoride
(PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO), and
mixtures thereof.
Alternatively, other than the liquid nonaqueous electrolyte and gel
nonaqueous electrolyte, a room temperature molten salt (ionic melt)
including lithium ions, a polymer solid electrolyte, an inorganic
solid electrolyte, or the like may be used as the nonaqueous
electrolyte.
The room temperature molten salt (ionic melt) indicates compounds
among organic salts made of combinations of organic cations and
anions, which are able to exist in a liquid state at room
temperature (15.degree. C. to 25.degree. C.). The room temperature
molten salt includes a room temperature molten salt which exists
alone, as a liquid, a room temperature molten salt which becomes a
liquid upon mixing with an electrolyte salt, a room temperature
molten salt which becomes a liquid when dissolved in an organic
solvent, and mixtures thereof. In general, the melting point of the
room temperature molten salt used in secondary batteries is
25.degree. C. or below. The organic cations generally have a
quaternary ammonium framework.
The polymer solid electrolyte is prepared by dissolving the
electrolyte salt in a polymeric material, and solidifying it.
The inorganic solid electrolyte is a solid substance having Li ion
conductivity.
(4) Separator
The separator may be made of, for example, a porous film or
synthetic resin nonwoven fabric including polyethylene (PE),
polypropylene (PP), cellulose, or polyvinylidene fluoride (PVdF).
In view of safety, a porous film made of polyethylene or
polypropylene is preferred. This is because such a porous film
melts at a fixed temperature and thus able to shut off current.
(5) Container Member
As the container member, for example, a container made of laminate
film or a container made of metal may be used.
The thickness of the laminate film is, for example, 0.5 mm or less,
and preferably 0.2 mm or less.
As the laminate film, used is a multilayer film including multiple
resin layers and a metal layer sandwiched between the resin layers.
The resin layer may include, for example, a polymeric material such
as polypropylene (PP), polyethylene (PE), nylon, or polyethylene
terephthalate (PET). The metal layer is preferably made of aluminum
foil or an aluminum alloy foil, so as to reduce weight. The
laminate film may be formed into the shape of a container member,
by heat-sealing.
The wall thickness of the metal container is, for example, 1 mm or
less, more preferably 0.5 mm or less, and still more preferably 0.2
mm or less.
The metal case is made, for example, of aluminum or an aluminum
alloy. The aluminum alloy preferably contains elements such as
magnesium, zinc, or silicon. If the aluminum alloy contains a
transition metal such as iron, copper, nickel, or chromium, the
content thereof is preferably 100 ppm by mass or less.
The shape of the container member is not particularly limited. The
shape of the container member may be, for example, flat (thin),
square, cylinder, coin, or button-shaped. The container member may
be appropriately selected depending on battery size and use of the
battery.
(6) Negative Electrode Terminal
The negative electrode terminal may be made of a material that is
electrochemically stable at the potential at which Li is inserted
into and extracted from the above-described negative electrode
active material, and has electrical conductivity. Specific examples
of the material for the negative electrode terminal include copper,
nickel, stainless steel, aluminum, and aluminum alloy containing at
least one element selected from the group consisting of Mg, Ti, Zn,
Mn, Fe, Cu, and Si. Aluminum or aluminum alloy is preferred as the
material for the negative electrode terminal. The negative
electrode terminal is preferably made of the same material as the
negative electrode current collector, in order to reduce the
contact resistance with the negative electrode current
collector.
(7) Positive Electrode Terminal
The positive electrode terminal may be made of, for example, a
material that is electrically stable in the potential range of 3 V
to 5 V (vs. Li/Li.sup.+) relative to the redox potential of
lithium, and has electrical conductivity. Examples of the material
for the positive electrode terminal include aluminum and an
aluminum alloy containing one or more selected from the group
consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. The positive
electrode terminal is preferably made of the same material as the
positive electrode current collector, in order to reduce contact
resistance with the positive electrode current collector.
Next, the secondary battery according to the third embodiment will
be more specifically described with reference to the drawings.
FIG. 4 is a cross-sectional view schematically showing an example
of a secondary battery according to the third embodiment. FIG. 5 is
an enlarged cross-sectional view of section A of the secondary
battery shown in FIG. 4.
The secondary battery 100 shown in FIGS. 4 and 5 includes a
bag-shaped container member 2 shown in FIGS. 4 and 5, an electrode
group 1 shown in FIG. 4, and an electrolyte, which is not shown.
The electrode group 1 and the electrolyte are housed in the
bag-shaped container member 2. The electrolyte (not shown) is held
in the electrode group 1.
The bag-shaped container member 2 is made of a laminate film
including two resin layers and a metal layer sandwiched between the
resin layers.
As shown in FIG. 4, the electrode group 1 is a wound electrode
group in a flat form. The wound electrode group 1 in a flat form
includes a negative electrode 3, a separator 4, and a positive
electrode 5, as shown in FIG. 5. The separator 4 is sandwiched
between the negative electrode 3 and the positive electrode 5.
The negative electrode 3 includes a negative electrode current
collector 3a and a negative electrode active material-containing
layer 3b. At the portion of the negative electrode 3 positioned
outermost among the wound electrode group 1, the negative electrode
active material-containing layer 3b is formed only on an inner
surface of the negative electrode current collector 3a, as shown in
FIG. 4. For the other portions of the negative electrode 3,
negative electrode active material-containing layers 3b are formed
on both of reverse surfaces of the negative electrode current
collector 3a.
The positive electrode 5 includes a positive electrode current
collector 5a and positive electrode active material-containing
layers 5b formed on both of reverse surfaces of the positive
electrode current collector 5a.
As shown in FIG. 4, a negative electrode terminal and positive
electrode terminal 7 are positioned in vicinity of the outer
peripheral edge of the wound electrode group 1. The negative
electrode terminal 6 is connected to a portion of the negative
electrode current collector 3a positioned outermost. The positive
electrode terminal 7 is connected to a portion of the positive
electrode current collector 5a positioned outermost. The negative
electrode terminal 6 and the positive electrode terminal 7 extend
out from an opening of the bag-shaped container member 2. A
thermoplastic resin layer is provided on the inner surface of the
bag-shaped container member 2, and the opening is sealed by
heat-sealing the resin layer.
The secondary battery according to the third embodiment is not
limited to the secondary battery of the structure shown in FIGS. 4
and 5, and may be, for example, a battery of a structure as shown
in FIGS. 6 and 7.
FIG. 6 is a partially cut-out perspective view schematically
showing another example of a secondary battery according to the
third embodiment. FIG. 7 is an enlarged cross-sectional view of
section B of the secondary battery shown in FIG. 6.
The secondary battery 100 shown in FIGS. 6 and 7 includes an
electrode group 1 shown in FIGS. 6 and 7, a container member 2
shown in FIG. 6, and an electrolyte, which is not shown. The
electrode group 1 and the electrolyte are housed in the container
member 2. The electrolyte is held in the electrode group 1.
The container member 2 is made of a laminate film including two
resin layers and a metal layer sandwiched between the resin
layers.
As shown in FIG. 7, the electrode group 1 is a stacked electrode
group. The stacked electrode group 1 has a structure in which and
negative electrodes 3 and positive electrodes 5 are alternately
stacked with separator(s) 4 sandwiched therebetween.
The electrode group 1 includes plural negative electrodes 3. Each
of the negative electrodes 3 includes the negative electrode
current collector 3a and the negative electrode active
material-containing layers 3b supported on both surfaces of the
negative electrode current collector 3a. The electrode group 1
further includes plural positive electrodes 5. Each of the positive
electrodes 5 includes the positive electrode current collector 5a
and the positive electrode active material-containing layers 5b
supported on both surfaces of the positive electrode current
collector 5a.
The negative electrode current collector 3a of each of the negative
electrodes 3 includes at one end, a portion 3c where the negative
electrode active material-containing layer 3b is not supported on
either surface. This portion 3c serves as a negative electrode tab.
As shown in FIG. 7, the portions 3c serving as the negative
electrode tabs do not overlap the positive electrodes 5. The plural
negative electrode tabs (portions 3c) are electrically connected to
the strip-shaped negative electrode terminal 6. A tip of the
strip-shaped negative electrode terminal 6 is drawn to the outside
from the container member 2.
Although not shown, the positive electrode current collector 5a of
each of the positive electrodes 5 includes at one end, a portion
where the positive electrode active material-containing layer 5b is
not supported on either surface. This portion serves as a positive
electrode tab. Like the negative electrode tabs (portion 3c), the
positive electrode tabs do not overlap the negative electrodes 3.
Further, the positive electrode tabs are located on the opposite
side of the electrode group 1 with respect to the negative
electrode tabs (portion 3c). The positive electrode tabs are
electrically connected to the strip-shaped positive electrode
terminal 7. A tip of the strip-shaped positive electrode terminal 7
is located on the opposite side relative to the negative electrode
terminal 6 and drawn to the outside from the container member
2.
The secondary battery according to the third embodiment includes
the active material according to the first embodiment as a negative
electrode active material. Therefore, the secondary battery can
exhibit excellent rate characteristics.
Fourth Embodiment
According to a fourth embodiment, a battery module is provided. The
battery module according to the fourth embodiment includes plural
secondary batteries according to the third embodiment.
In the battery module according to the fourth embodiment, each of
the single batteries may be arranged electrically connected in
series, in parallel, or in a combination of in-series connection
and in-parallel connection.
An example of the battery module according to the fourth embodiment
will be described next with reference to the drawings.
FIG. 8 is a perspective view schematically showing an example of
the battery module according to the fourth embodiment. A battery
module 200 shown in FIG. 8 includes five single-batteries 100a to
100e, four bus bars 21, a positive electrode-side lead 22, and a
negative electrode-side lead 23. Each of the five single-batteries
100a to 100e is a secondary battery according to the third
embodiment.
The bus bar 21 connects, for example, a negative electrode terminal
6 of one single-battery 100a and a positive electrode terminal 7 of
the single-battery 100b positioned adjacent. In such a manner, five
single-batteries 100 are thus connected in series by the four bus
bars 21. That is, the battery module 200 shown in FIG. 8 is a
battery module of five in-series connection.
As shown in FIG. 8, the positive electrode terminal 7 of the
single-battery 100a located at left end among the five
single-batteries 100a to 100e is connected to the positive
electrode-side lead 22 for external connection. In addition, the
negative electrode terminal 6 of the single-battery 100e located at
the right end among the five single-batteries 100a to 100e is
connected to the negative electrode-side lead 23 for external
connection.
The battery module according to the fourth embodiment includes the
secondary battery according to the third embodiment. Therefore, the
battery module can exhibit excellent rate characteristics.
Fifth Embodiment
According to a fifth embodiment, a battery pack is provided. The
battery pack includes a battery module according to the fourth
embodiment. The battery pack may include a single secondary battery
according to the third embodiment, in place of the battery module
according to the fourth embodiment.
The battery pack according to the fifth embodiment may further
include a protective circuit. The protective circuit has a function
to control charging and discharging of the secondary battery.
Alternatively, a circuit included in equipment where the battery
pack serves as a power source (for example, electronic devices,
vehicles, and the like) may be used as the protective circuit for
the battery pack.
Moreover, the battery pack according to the fifth embodiment may
further include an external power distribution terminal. The
external power distribution terminal is configured to externally
output current from the secondary battery, and to input external
current into the secondary battery. In other words, when the
battery pack is used as a power source, the current is provided out
via the external power distribution terminal. When the battery pack
is charged, the charging current (including regenerative energy of
motive force of vehicles such as automobiles) is provided to the
battery pack via the external power distribution terminal.
Next, an example of a battery pack according to the fifth
embodiment will be described with reference to the drawings.
FIG. 9 is an exploded perspective view schematically showing an
example of the battery pack according to the fifth embodiment. FIG.
10 is a block diagram showing an example of an electric circuit of
the battery pack shown in FIG. 9.
A battery pack 300 shown in FIGS. 9 and 10 includes a housing
container 31, a lid 32, protective sheets 33, a battery module 200,
a printed wiring board 34, wires 35, and an insulating plate (not
shown).
The housing container 31 shown in FIG. 9 is a square bottomed
container having a rectangular bottom surface. The housing
container 31 is configured to be capable of storing the protection
sheets 33, the battery module 200, the printed wiring board 34, and
the wires 35. The lid 32 has a rectangular shape. The lid 32 covers
the housing container 31 to house the battery module 200 and such.
The housing container 31 and the lid 32 are provided with openings,
connection terminals, or the like (not shown) for connection to an
external device or the like.
The battery module 200 includes plural single-batteries 100, a
positive electrode-side lead 22, a negative electrode-side lead 23,
and adhesive tape(s) 24.
A single-battery 100 has a structure shown in FIGS. 4 and 5. At
least one of the plural single-batteries 100 is a secondary battery
according to the third embodiment. The plural single-batteries 100
are stacked such that the negative electrode terminals 6 and the
positive electrode terminals 7, which extend outside, are directed
toward the same direction. The plural single-batteries 100 are
electrically connected in series, as shown in FIG. 9. The plural
single-batteries 100 may alternatively be electrically connected in
parallel, or connected in a combination of in-series connection and
in-parallel connection. If the plural single-batteries 100 are
connected in parallel, the battery capacity increases as compared
to a case in which they are connected in series.
The adhesive tape(s) 24 fastens the plural single-batteries 100.
The plural single-batteries 100 may be fixed using a
heat-shrinkable tape in place of the adhesive tape(s) 24. In this
case, the protective sheets 33 are arranged on both side surfaces
of the battery module 200, and the heat-shrinkable tape is wound
around the battery module 200 and protective sheets 33. After that,
the heat-shrinkable tape is shrunk by heating to bundle the plural
single-batteries 100.
One end of the positive electrode-side lead 22 is connected to the
positive electrode terminal 7 of the single-battery 100 located
lowermost in the stack of the single-batteries 100. One end of the
negative electrode-side lead 23 is connected to the negative
electrode terminal 6 of the single-battery 100 located uppermost in
the stack of the single-batteries 100.
The printed wiring board 34 is provided along one face in the
short-side direction among the inner surfaces of the housing
container 31. The printed wiring board 34 includes a positive
electrode-side connector 341, a negative electrode-side connector
342, a thermistor 343, a protective circuit 344, wirings 345 and
346, an external power distribution terminal 347, a plus-side
(positive-side) wire 348a, and a minus-side (negative-side) wire
348b. One principal surface of the printed wiring board 34 faces
the surface of the battery module 200 from which the negative
electrode terminals 6 and the positive electrode terminals 7 extend
out. An insulating plate (not shown) is disposed in between the
printed wiring board 34 and the battery module 200.
The positive electrode-side connector 341 is provided with a
through hole. By inserting the other end of the positive
electrode-side lead 22 into the though hole, the positive
electrode-side connector 341 and the positive electrode-side lead
22 become electrically connected. The negative electrode-side
connector 342 is provided with a through hole. By inserting the
other end of the negative electrode-side lead 23 into the though
hole, the negative electrode-side connector 342 and the negative
electrode-side lead 23 become electrically connected.
The thermistor 343 is fixed to one principal surface of the printed
wiring board 34. The thermistor 343 detects the temperature of each
single-battery 100 and transmits detection signals to the
protective circuit 344.
The external power distribution terminal 347 is fixed to the other
principal surface of the printed wiring board 34. The external
power distribution terminal 347 is electrically connected to
device(s) that exists outside the battery pack 300.
The protective circuit 344 is fixed to the other principal surface
of the printed wiring board 34. The protective circuit 344 is
connected to the external power distribution terminal 347 via the
plus-side wire 348a. The protective circuit 344 is connected to the
external power distribution terminal 347 via the minus-side wire
348b. In addition, the protective circuit 344 is electrically
connected to the positive electrode-side connector 341 via the
wiring 345. The protective circuit 344 is electrically connected to
the negative electrode-side connector 342 via the wiring 346.
Furthermore, the protective circuit 344 is electrically connected
to each of the plural single-batteries 100 via the wires 35.
The protective sheets 33 are arranged on both inner surfaces of the
housing container 31 along the long-side direction and on the inner
surface along the short-side direction facing the printed wiring
board 34 across the battery module 200 positioned therebetween. The
protective sheets 33 are made of, for example, resin or rubber.
The protective circuit 344 controls charge and discharge of the
plural single-batteries 100. The protective circuit 344 is also
configured to cut-off electric connection between the protective
circuit 344 and the external power distribution terminal 347 to
external device(s), based on detection signals transmitted from the
thermistor 343 or detection signals transmitted from each
single-battery 100 or the battery module 200.
An example of the detection signal transmitted from the thermistor
343 is a signal indicating that the temperature of the
single-battery (single-batteries) 100 is detected to be a
predetermined temperature or more. An example of the detection
signal transmitted from each single-battery 100 or the battery
module 200 include a signal indicating detection of over-charge,
over-discharge, and overcurrent of the single-battery
(single-batteries) 100. When detecting over-charge or the like for
each of the single batteries 100, the battery voltage may be
detected, or a positive electrode potential or negative electrode
potential may be detected. In the latter case, a lithium electrode
to be used as a reference electrode may be inserted into each
single battery 100.
Note, that as the protective circuit 344, a circuit included in a
device (for example, an electronic device or an automobile) that
uses the battery pack 300 as a power source may be used.
As described above, the battery pack 300 includes the external
power distribution terminal 347. Hence, the battery pack 300 can
output current from the battery module 200 to an external device
and input current from an external device to the battery module 200
via the external power distribution terminal 347. In other words,
when using the battery pack 300 as a power source, the current from
the battery module 200 is supplied to an external device via the
external power distribution terminal 347. When charging the battery
pack 300, a charge current from an external device is supplied to
the battery pack 300 via the external power distribution terminal
347. If the battery pack 300 is used as an onboard battery, the
regenerative energy of the motive force of a vehicle can be used as
the charge current from the external device.
Note that the battery pack 300 may include plural battery modules
200. In this case, the plural battery modules 200 may be connected
in series, in parallel, or connected in a combination of in-series
connection and in-parallel connection. The printed wiring board 34
and the wires 35 may be omitted. In this case, the positive
electrode-side lead 22 and the negative electrode-side lead 23 may
be used as the external power distribution terminal.
Such a battery pack is used, for example, in applications where
excellent cycle performance is demanded when a large current is
extracted. More specifically, the battery pack is used as, for
example, a power source for electronic devices, a stationary
battery, or an onboard battery for various kinds of vehicles. An
example of the electronic device is a digital camera. The battery
pack is particularly favorably used as an onboard battery.
The battery pack according to the fifth embodiment includes the
secondary battery according to the third embodiment or the battery
module according to the fourth embodiment. Therefore, the battery
pack can exhibit excellent rate characteristics.
Sixth Embodiment
According to a sixth embodiment, a vehicle is provided. The vehicle
includes the battery pack according to the fifth embodiment.
In the vehicle according to the sixth embodiment, the battery pack
is configured, for example, to recover regenerative energy from
motive force of the vehicle. The vehicle may include a mechanism
configured to convert kinetic energy of the vehicle into
regenerative energy.
Examples of the vehicle according to the sixth embodiment include
two-wheeled to four-wheeled hybrid electric automobiles,
two-wheeled to four-wheeled electric automobiles, electrically
assisted bicycles, and railway cars.
In the vehicle according to the sixth embodiment, the installing
position of the battery pack is not particularly limited. For
example, the battery pack may be installed in the engine
compartment of the vehicle, in rear parts of the vehicle, or under
seats.
The vehicle according to the sixth embodiment may have plural
battery packs installed. In such a case, the battery packs may be
electrically connected in series, electrically connected in
parallel, or electrically connected in a combination of in-series
connection and in-parallel connection.
An example of the vehicle according to the sixth embodiment is
explained below, with reference to the drawings.
FIG. 11 is a cross-sectional view schematically showing an example
of a vehicle according to the sixth embodiment.
A vehicle 400, shown in FIG. 11 includes a vehicle body 40 and a
battery pack 300 according to the fifth embodiment. In the example
shown in FIG. 11, the vehicle 400 is a four-wheeled automobile.
The vehicle 400 may have plural battery packs 300 installed. In
such a case, the battery packs 300 may be connected in series,
connected in parallel, or connected in a combination of in-series
connection and in-parallel connection.
In FIG. 11, the battery pack 300 is installed in an engine
compartment located at the front of the vehicle body 40. As
mentioned above, for example, the battery pack 300 may be
alternatively installed in rear sections of the vehicle body 40, or
under a seat. The battery pack 300 may be used as a power source of
the vehicle 400. The battery pack 300 can also recover regenerative
energy of motive force of the vehicle 400.
Next, with reference to FIG. 12, an aspect of operation of the
vehicle according to the sixth embodiment is explained.
FIG. 12 is a view schematically showing another example of the
vehicle according to the sixth embodiment. A vehicle 400, shown in
FIG. 12, is an electric automobile.
The vehicle 400, shown in FIG. 12, includes a vehicle body 40, a
vehicle power source 41, a vehicle ECU (electric control unit) 42,
which is a master controller of the vehicle power source 41, an
external terminal (an external power connection terminal) 43, an
inverter 44, and a drive motor 45.
The vehicle 400 includes the vehicle power source 41, for example,
in the engine compartment, in the rear sections of the automobile
body, or under a seat. In FIG. 12, the position of the vehicle
power source 41 installed in the vehicle 400 is schematically
shown.
The vehicle power source 41 includes plural (for example, three)
battery packs 300a, 300b and 300c, battery management unit (EMU)
411, and a communication bus 412.
The three battery packs 300a, 300b and 300c are electrically
connected in series. The battery pack 300a includes a battery
module 200a and a battery module monitoring unit 301a (e.g., a VTM:
voltage temperature monitoring). The battery pack 300b includes a
battery module 200b, and a battery module monitoring unit 301b. The
battery pack 300c includes a battery module 200c, and a battery
module monitoring unit 301c. The battery packs 300a, 300b and 300c
can each be independently removed, and may be exchanged by a
different battery pack 300.
Each of the battery modules 200a to 200c includes plural
single-batteries connected in series. At least one of the plural
single-batteries is the secondary battery according to the third
embodiment. The battery modules 200a to 200c each perform charging
and discharging via a positive electrode terminal 413 and a
negative electrode terminal 414.
In order to collect information concerning security of the vehicle
power source 41, the battery management unit 411 performs
communication with the battery module monitoring units 301a to 301c
and collects information such as voltages or temperatures of the
single-batteries 100 included in the battery modules 200a to 200c
included in the vehicle power source 41.
The communication bus 412 is connected between the battery
management unit 411 and the battery module monitoring units 301a to
301c. The communication bus 412 is configured so that multiple
nodes (i.e., the battery management unit and one or more battery
module monitoring units) share a set of communication lines. The
communication bus 412 is, for example, a communication bus
configured based on CAN (Control Area Network) standard.
The battery module monitoring units 301a to 301c measure a voltage
and a temperature of each single-battery in the battery modules
200a to 200c based on commands from the battery management unit
411. It is possible, however, to measure the temperatures only at
several points per battery module, and the temperatures of all of
the single-batteries need not be measured.
The vehicle power source 41 may also have an electromagnetic
contactor (for example, a switch unit 415 shown in FIG. 12) for
switching connection between the positive electrode terminal 413
and the negative electrode terminal 414. The switch unit 415
includes a precharge switch (not shown), which is turned on when
the battery modules 200a to 200c are charged, and a main switch
(not shown), which is turned on when battery output is supplied to
a load. The precharge switch and the main switch include a relay
circuit (not shown), which is turned on or off based on a signal
provided to a coil disposed near the switch elements.
The inverter 44 converts an inputted direct current voltage to a
three-phase alternate current (AC) high voltage for driving a
motor. Three-phase output terminal(s) of the inverter 44 is (are)
connected to each three-phase input terminal of the drive motor 45.
The inverter 44 controls an output voltage based on control signals
from the battery management unit 411 or the vehicle ECU 42, which
controls the entire operation of the vehicle.
The drive motor 45 is rotated by electric power supplied from the
inverter 44. The rotation is transferred to an axle and driving
wheels W via a differential gear unit, for example.
The vehicle 400 also includes a regenerative brake mechanism
(regenerator), though not shown. The regenerative brake mechanism
rotates the drive motor 45 when the vehicle 400 is braked, and
converts kinetic energy into regenerative energy, as electric
energy. The regenerative energy, recovered in the regenerative
brake mechanism, is inputted into the inverter 44 and converted to
direct current. The direct current is inputted into the vehicle
power source 41.
One terminal of a connecting line L1 is connected via a current
detector (not shown) in the battery management unit 411 to the
negative electrode terminal 414 of the vehicle power source 41. The
other terminal of the connecting line L1 is connected to a negative
electrode input terminal of the inverter 44.
One terminal of a connecting line L2 is connected via the switch
unit 415 to the positive electrode terminal 413 of the vehicle
power source 41. The other terminal of the connecting line L2 is
connected to a positive electrode input terminal of the inverter
44.
The external terminal 43 is connected to the battery management
unit 411. The external terminal 43 is able to connect, for example,
to an external power source.
The vehicle ECU 42 cooperatively controls the battery management
unit 411 together with other units in response to inputs operated
by a driver or the like, thereby performing the management of the
whole vehicle. Data concerning the security of the vehicle power
source 41, such as a remaining capacity of the vehicle power source
41, are transferred between the battery management unit 411 and the
vehicle ECU 42 via communication lines.
The vehicle according to the sixth embodiment includes the battery
pack according to the fifth embodiment. Therefore, according to the
present embodiment, it is possible to provide a vehicle including a
battery pack that can exhibit excellent rate characteristics.
EXAMPLES
Hereinafter, the above embodiment will be described in more detail
based on examples.
Example 1
(Preparation of Active Material Particles)
First, titanium dioxide, niobium pentoxide, and phosphorus
pentoxide were mixed at a molar ratio of 1.0000:0.9995:0.000525,
respectively, and a raw material mixed powder was prepared using a
ball mill. Next, this mixture was placed in a gold boat (boat made
of gold) and pre-fired at 350.degree. C. for 2 hours. Next, the
powder after the pre-firing was transferred to a platinum crucible
and used for first main firing. The first main firing was performed
at 800.degree. C. for 12 hours. Thereafter, the obtained powder was
ground again using a ball mill for 1 hour. This powder was placed
in a platinum crucible, subjected to second main firing, and then
rapidly quenched with liquid nitrogen to obtain an active material
according to Example 1. The obtained active material particles
contained primary particles and secondary particles. The second
main firing and rapidly quenching treatment was performed by firing
at a firing temperature of 1000.degree. C. for 2 hours with a
temperature rise rate of 10.degree. C./min and then taken out of an
electric furnace promptly and placed into the liquid nitrogen
together with the platinum crucible.
(Formation of Carbon-Containing Layer on Particle Surface)
Next, the active material particles obtained by the above-described
method were loaded with a carbon body to obtain an active material
powder having a carbon-containing layer on the primary particles
surface and the secondary particle surfaces. Specifically, first,
polyvinyl alcohol (PVA) and pure water were mixed to prepare a PVA
aqueous solution. A concentration of PVA in the PVA aqueous
solution was 15% by mass. Subsequently, the active material
particles were added to the aqueous PVA solution and stirred to
prepare a dispersion liquid. Next, this dispersion liquid was
subjected to spray drying to obtain a powder sample. This powder
sample was further dried at a temperature of 100.degree. C. for 12
hours to obtain active material particles supporting unfired carbon
bodies. Subsequently, the active material particles were subjected
to carbonization treatment in a reducing atmosphere at a
temperature of 700.degree. C. for 1 hour to obtain an active
material powder having a carbon-containing layer on the particle
surfaces.
(Preparation of Negative Electrode)
A negative electrode was prepared as follows.
First, 100 parts by mass of active material particles, 6 parts by
mass of a conductive agent and 4 parts by mass of a binder were
dispersed in a solvent to prepare a slurry. As the active material
particles, an active material powder obtained by the
above-described method, and having a carbon-containing layer on the
particle surface was used. As the conductive agent, a mixture of
acetylene black and graphite was used. In this mixture, a mass
ratio of acetylene black and graphite was 1:2. As the binder, a
mixture of carboxyl methyl cellulose (CMC) and styrene butadiene
rubber (SBR) was used. In this mixture, a mass ratio of CMC and SBR
was 1:1. As the solvent, pure water was used.
Subsequently, the obtained slurry was applied to both sides of the
current collector, and the coating film was dried to form an active
material-containing layer. As the current collector, an aluminum
foil having a thickness of was used. This was dried under vacuum at
130.degree. C. for 12 hours, and the current collector and the
active material-containing layer were rolled with a roll press
machine to obtain a negative electrode. Press pressure was made
common to Examples and Comparative Examples.
In order to measure a unipolar capacity at the negative electrode,
a three-electrode beaker cell was prepared by using the electrode
(negative electrode) obtained by the above-described method as a
working electrode, metallic lithium foil as a counter electrode and
a reference electrode, and using a nonaqueous electrolyte prepared
by a method described late.
In this three-electrode beaker cell for measurement, the lithium
metal is used as the counter electrode, so that the potential of
the electrode (negative electrode) produced in each of Examples and
Comparative Examples is nobler than that of the counter electrode.
Hence the prepared electrode (negative electrode) operates as a
positive electrode. Therefore, the definitions of charge and
discharge become opposite when the electrodes of each of Examples
and Comparative Example is used as the negative electrode. Here, in
the present examples, in order to avoid confusion, directions in
which lithium ions are inserted into the electrode are collectively
referred to as charge, and directions in which lithium ions are
extracted are consistently referred to as discharge. Note that the
active material of the present embodiment operates as a negative
electrode by combining with a known positive electrode
material.
(Preparation of Nonaqueous Electrolyte)
As a mixed solvent, a mixed solvent of ethylene carbonate and
diethyl carbonate (volume ratio 1:1) was prepared. A nonaqueous
electrolyte was prepared by dissolving lithium hexafluorophosphate
(LiPF.sub.6) at a concentration of 1M in this solvent.
Example 2
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of
1.000:0.995:0.05, respectively, and the raw material mixed powder
was prepared using a ball mill. The obtained active material
particles contained primary particles and secondary particles.
Thereafter, a three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Example 3
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of
1.000:0.995:0.25, respectively, and the raw material mixed powder
was prepared using a ball mill. The obtained active material
particles contained primary particles and secondary particles.
Thereafter, a three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Example 4
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of 1.000:0.995:0.5
and a raw material mixed powder was prepared using a ball mill. The
obtained active material particles contained primary particles and
secondary particles. Thereafter, a three-electrode beaker cell was
prepared in the same manner as described in Example 1 except that
the above active material was used.
Example 5
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of
1.000:0.975:0.53, respectively, and the raw material mixed powder
was prepared using a ball mill. The obtained active material
particles contained primary particles and secondary particles.
Thereafter, a three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Example 6
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of
1.000:0.95:0.25, respectively, and the raw material mixed powder
was prepared using a ball mill. The obtained active material
particles contained primary particles and secondary particles.
Thereafter, a three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Example 7
Titanium dioxide, niobium pentoxide, and phosphorus pentoxide were
mixed at a molar ratio of 1.000:0.95:0.25, respectively, and a raw
material mixed powder was prepared using a ball mill. This mixture
was placed in a gold boat and pre-fired at 350.degree. C. for 2
hours, and then the resultant powder was cooled to room
temperature. Next, potassium carbonate was added to the resultant
powder so as to be equimolar to the phosphorus pentoxide used as a
raw material, and the powder was mixed in a ball mill to prepare a
powder. This powder was subjected to the first main firing, the
second main firing, and the rapidly quenching treatment in the same
manner as described in Example 1 to further form a
carbon-containing layer and obtain an active material according to
Example 7. The obtained active material particles contained primary
particles and secondary particles. A three-electrode beaker cell
was prepared in the same manner as described in Example 1 except
that the above active material was used.
Example 8
An active material was prepared in the same manner as described in
Example 7 except that ferric oxide was added instead of potassium
carbonate as a powder to be added after the pre-firing. The
obtained active material particles contained primary particles and
secondary particles. A three-electrode beaker cell was prepared in
the same manner as described in Example 1 except that the above
active material was used.
Example 9
An active material was obtained in the same manner as described in
Example 1 except that titanium dioxide, niobium pentoxide, and
phosphorus pentoxide were mixed at a molar ratio of
1.000:0.75:1.25, respectively, and a raw material mixed powder was
prepared using a ball mill. The obtained active material particles
contained primary particles and secondary particles. Thereafter, a
three-electrode beaker cell was prepared in the same manner as
described in Example 1 except that the above active material was
used.
Example 10
An active material was obtained in the same manner as described in
Example 7 except that as raw material mixed powder, titanium
dioxide, niobium pentoxide, tantalum pentoxide, vanadium pentoxide,
bismuth (ITT) oxide, and phosphorus pentoxide were added so that
the molar ratio was 1.00:0.98:0.005:0.005:0.005:0.01. The obtained
active material particles contained primary particles and secondary
particles. Thereafter, a three-electrode beaker cell was prepared
in the same manner as described in Example 1 except that the above
active material was used.
Example 11
An active material was obtained in the same manner as described in
Example 7 except that titanium dioxide, niobium pentoxide,
potassium carbonate, silicon oxide and phosphorus pentoxide were
mixed as raw material mixed powders so that the respective molar
ratios were 0.95:0.99:0.005:0.01:0.05. The obtained active material
particles contained primary particles and secondary particles.
Thereafter, a three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Example 12
An active material was obtained in the same manner as described in
Example 7 except that as a raw material mixed powder, titanium
dioxide, niobium pentoxide, molybdenum trioxide, sodium carbonate,
magnesium oxide, tungsten trioxide, and phosphorus pentoxide were
mixed in a molar ratio of 0.93:0.985:0.02:0.005:0.01:0.03:0.0125.
The obtained active material particles contained primary particles
and secondary particles. Thereafter, a three-electrode beaker cell
was prepared in the same manner as described in Example 1 except
that the above active material was used.
Example 13
An active material was obtained in the same manner as described in
Example 7 except that as a raw material mixed powder, titanium
dioxide, niobium pentoxide, molybdenum trioxide, chromium trioxide,
iron (III) oxide, aluminum oxide, boron oxide and phosphorus
pentoxide were mixed in molar ratios of
0.7:0.845:0.15:0.025:0.025:0.05:0.05:0.015 was used. The obtained
active material particles contained primary particles and secondary
particles. Thereafter, a three-electrode beaker cell was prepared
in the same manner as described in Example 1 except that the above
active material was used.
Comparative Example 1
Titanium dioxide and niobium pentoxide were mixed at a molar ratio
of 1:1, respectively, and a raw material mixed powder was prepared
using a ball mill. Next, this mixture was heated at a firing
temperature of 1200.degree. C. for 12 hours to obtain an active
material according to Comparative Example 1. The obtained active
material particles contained primary particles and secondary
particles. A three-electrode beaker cell was prepared in the same
manner as described in Example 1 except that the above active
material was used.
Comparative Example 2
An active material according to Comparative Example 2 was obtained
in the same manner as described in Example 1 except that in the
second main firing, the temperature was raised at a rate of
10.degree. C./min and fired for 10 hours at a firing temperature of
1000.degree. C., then the temperature of the electric furnace was
decreased at a rate of 10.degree. C./min and slowly cooled to room
temperature. The obtained active material particles contained
primary particles and secondary particles. A three-electrode beaker
cell was prepared in the same manner as described in Example 1
except that the above active material was used.
Comparative Example 3
Titanium dioxide, niobium pentoxide, and phosphorus pentoxide were
mixed at a molar ratio of 1:0.9:0.1, respectively, and a raw
material mixed powder was prepared using a ball mill. Next, this
mixture was heated at a firing temperature of 1200.degree. C. for
12 hours to obtain an active material represented by the chemical
formula Nb.sub.1.8P.sub.0.2TiO.sub.7. Next, titanium
tetraisopropoxide, niobium chloride, and phosphoric acid were mixed
in ethanol at a molar ratio of 1:0.95:0.05 to prepare a sol-gel
solution. The sol-gel solution was spray-coated on the surface of
the active material powder represented by the chemical formula
Nb.sub.1.8P.sub.0.2TiO.sub.7 by using a tumbling fluidized device,
and then fired at 800.degree. C. for 5 hours, to form a layer
represented by the chemical formula Nb.sub.1.9P.sub.0.1TiO.sub.7.
The Nb.sub.1.9P.sub.0.1TiO.sub.7 layer obtained at this time was
30% by weight based on the Nb.sub.1.8P.sub.0.2TiO.sub.7 particles
as the core layer. This composite particle was used as an active
material of Comparative Example 3. The obtained active material
particles contained primary particles and secondary particles. A
three-electrode beaker cell was prepared in the same manner as
described in Example 1 except that the above active material was
used.
Comparative Example 4
The surfaces of the particles of the active material powder
obtained in Comparative Example 1 was coated with a lithium
phosphate compound according to the following procedure. First,
phosphoric acid was dissolved in a mixed solution of titanium
tetraisopropoxide and absolute ethanol with stirring to obtain a
sol-gel solution containing P and Li. The molar ratio of P and Li
in this sol-gel solution was P:Li=1:3. In addition, this solution
contained 30% by weight of a solid content calculated as solid
content of phosphoric acid and titanium tetraisopropoxide. Using
tumbling fluidized device, this sol-gel solution was coated on the
particle surfaces of the core particles of Comparative Example 1 to
obtain a precursor. This precursor was subjected to firing in air
at 400.degree. C. for 1 hour. Thus, an active material of
Comparative Example 4 was obtained. The obtained active material
particles contained primary particles and secondary particles. A
three-electrode beaker cell was prepared in the same manner as
described in Example 1 except that the above active material was
used.
<Various Analyses>
Powder X-ray diffraction and ICP analysis were performed on the
active materials obtained in Examples and Comparative Examples by
the method described in the first embodiment to specify an average
composition of the active materials and a phosphate compound
present on the primary particles surfaces. In addition, TEM-EDX
observation was performed by the method described in the first
embodiment, and a concentration ratio (C2/C1) was calculated. Table
1 shows the result thereof.
<Evaluation of Battery Characteristics>
Charge/discharge characteristics of the beaker cells according to
Examples and Comparative Examples were evaluated. For the
charge/discharge capacity, an initial capacity (0.2 C discharge
capacity) was measured when the battery was charged at a time
charge rate of 0.2 C for 10 hours and discharged at a time
discharge rate of 0.2 C. Also, a resistance of 1 kHz was measured
after the initial charge and discharge, and this value was taken as
an initial resistance value.
Next, the battery was charged at a time charge rate of 0.2 C for 10
hours, then discharged at a time discharge rate of 3 C, and a
capacity at this time was divided by the initial capacity to
calculate a 3 C/0.2 C discharge capacity ratio. The 3 C/0.2 C
discharge capacity ratio is an index of the rate
characteristics.
Further, a charge-and-discharge cycle was performed 100 times at a
time charge/discharge rate of 1 C, and the discharge capacity at a
charge/discharge rate of 0.2 C after 100 cycles was measured. The
value of this discharge capacity divided by the value of the
initial capacity and multiplied by 100 was taken as a cycle
capacity retention ratio. The capacity retention ratio is an index
of the life characteristics. Values each obtained by measuring a
resistance of 1 kHz after 100 cycles and dividing the resistance by
the initial resistance value as resistance ratios, and these values
were shown in Table 1. The resistance ratio is an index of the life
characteristics.
TABLE-US-00001 TABLE 1 Manufacturing Active material method
Phosphate Concen- Concen- Concen- Rapidly compound at tration
tration tration Synthesis quenching primary particle C1 C2 ratio
Concentration method treatment Chemical formula surface (atm %)
(atm %) C2/C1 gradient Example 1 1 Yes
Nb.sub.1.999P.sub.0.001TiO.sub.7 Phosphorus oxide 0.01 0.0105 1.05
Yes Example 2 1 Yes Nb.sub.1.99P.sub.0.01TiO.sub.7 Phosphorus oxide
0.10 0.98 9.80 Yes Example 3 1 Yes Nb.sub.1.99P.sub.0.01TiO.sub.7
Phosphorus oxide 0.10 5.2 52.00 Yes Example 4 1 Yes
Nb.sub.1.999P.sub.0.01TiO.sub.7 Phosphorus oxide 0.10 9.98 99.80
Yes Example 5 1 Yes Nb.sub.1.95P.sub.0.05TiO.sub.7 Phosphorus oxide
0.50 9.97 19.90 Yes Example 6 1 Yes Nb.sub.1.9P.sub.0.1TiO.sub.7
Phosphorus oxide 1.00 5.3 5.30 Yes Example 7 2 Yes
Nb.sub.1.9P.sub.0.1TiO.sub.7 Potassium 1.00 5.4 5.40 Yes phosphate
Example 8 2 Yes Nb.sub.1.9P.sub.0.1TiO.sub.7 Iron phosphate 1.00
5.1 5.10 Yes Example 9 1 Yes Nb.sub.1.5P.sub.0.5TiO.sub.7
Phosphorus oxide 5.10 15 3.02 Yes Example 10 2 Yes
Nb.sub.1.96P.sub.0.01Ta.sub.0.01V.sub.0.01Bi.sub.0.01TiO.- sub.7
Potassium 0.10 0.21 2.10 Yes phosphate Example 11 2 Yes
Nb.sub.1.98K.sub.0.01Ti.sub.0.95P.sub.0.06Si.sub.0.01O.su- b.7
Potassium 0.49 1.08 2.20 Yes phosphate Example 12 2 Yes
Nb.sub.1.97P.sub.0.01Na.sub.0.01Mg.sub.0.01Ti.sub.0.93Mo.-
sub.0.04W.sub.0.03O.sub.7 Potassium 0.10 0.25 2.50 Yes phosphate
Example 13 2 Yes
Nb.sub.1.69P.sub.0.01Mo.sub.0.3Ti.sub.0.7Cr.sub.0.05Fe.su-
b.0.05Al.sub.0.1B.sub.0.1O.sub.7 Potassium 0.10 0.29 2.90 Yes
phosphate Comparative -- No Nb.sub.2TiO.sub.7 N/A 0.00 0.00 -- No
Example 1 Comparative -- No Nb.sub.1.999P.sub.0.001TiO.sub.7
Phosphorus oxide 0.01 0.01 1.00 No Example 2 Comparative -- No
Nb.sub.1.8P.sub.0.2TiO.sub.7/Nb.sub.1.9P.sub.0.1TiO.sub.- 7
Phosphorus oxide 2.10 1.05 0.53 Inverse Example 3 Comparative -- No
Nb.sub.2TiO.sub.7 Potassium 0.00 0.00 -- No Example 4 phosphate
Battery characteristics 0.2 C Capacity discharge 3 C/0.2 C
retention ratio capacity discharge after 100 cycles Resistance
ratio (mAh/g) capacity ratio (%) after 100 cycles Example 1 270.5
0.92 98.5 1.1 Example 2 269.2 0.93 98.4 1.08 Example 3 264.3 0.9
98.5 1.07 Example 4 262.1 0.89 97.3 1.24 Example 5 266.5 0.89 98.4
1.07 Example 6 267 0.91 98.2 1.09 Example 7 267.2 0.9 99.1 1.03
Example 8 267.4 0.92 98.9 1.04 Example 9 261.9 0.87 97.8 1.15
Example 10 264.2 0.93 99.2 1.02 Example 11 265.8 0.93 99.4 1.03
Example 12 265.1 0.94 99 1.03 Example 13 266.7 0.94 99.2 1.02
Comparative 258.1 0.79 84.3 1.95 Example 1 Comparative 268.9 0.81
89.7 1.31 Example 2 Comparative 259.7 0.77 81.2 2.05 Example 3
Comparative 254.4 0.76 84.4 1.89 Example 4
In Table 1, the column of "Synthesis method" shows which is a
corresponding synthesis method between the first and second
synthesis methods described in the first embodiment. In the column
of "Phosphate compound at primary particle surface", the type of
the phosphate compound mainly present on the primary particle
surface is shown. In the column of "Concentration gradient", "Yes"
is shown for an example that includes a primary particle having a
concentration gradient in which the phosphorus concentration
increases from the position of the gravity point of the primary
particle toward the particle surface. "No" is shown for an example
that includes a primary particle having no concentration gradient.
In addition, "Inverse" is shown for an example that includes a
primary particle having a concentration gradient in which the
phosphorus concentration decreases from the position of the gravity
point of the primary particle toward the particle surface.
The following can be found from Table 1.
In each of Examples 1 to 13 including the primary particle having a
concentration gradient in which the phosphorus concentration
increases from the position of the gravity point of the primary
particle toward the particle surface, the rate characteristics were
excellent while a practical battery capacity was maintained.
Comparative Example 1 and Comparative Example 4 are monoclinic
niobium-titanium composite oxides not containing phosphorus. The
active material according to Comparative Example 4 further has
lithium phosphate as a phosphate compound on the primary particle
surface. These active materials were inferior in discharge capacity
and rate characteristics to Examples 1 to 13.
Comparative Example 2 is a monoclinic niobium-titanium composite
oxide containing phosphorus, but the phosphorus concentration at
the position of the gravity point of primary particles is the same
as the phosphorus concentration at the position corresponding to
80% of the length defined from the gravity point to the surface of
the primary particle. That is, the active material according to
Comparative Example 2 does not have a concentration gradient of
phosphorus. The discharge capacity of the active material according
to Comparative Example 2 was relatively excellent, but the rate
characteristics thereof were inferior to those of Examples 1 to
13.
Comparative Example 3 contained a primary particle having a
concentration gradient in which the phosphorus concentration
gradually decreases from the position of the gravity point of the
primary particle toward the particle surface. Similarly to
Comparative Examples 1 and 4, Comparative Example 3 was inferior to
Examples 1 to 13 in both capacity and rate characteristics.
In Examples 1 to 13 in which the concentration ratio C2/C1 was in
the range of 1.05 to 100, the rate characteristics were excellent
while the practical battery capacity was maintained, as compared
with Comparative Examples 1 to 4. The reason for this is considered
to be that the diffusion rate of lithium ions is high in the
vicinity of the surface of the particle having a high phosphorus
concentration and the relatively large amount of lithium ions can
be inserted in the vicinity of the gravity point of the particle
having a low phosphorus concentration.
In addition, the active material produced in each of Examples 1 to
13 contained a secondary particle formed of a plurality of primary
particles, and a phosphate compound was present between these
primary particles. The capacity retention ratios and resistance
ratios of Examples 1 to 13 were superior to those of Comparative
Examples. This is considered to be because the lithium conduction
path could be maintained by suppressing the shredding of the
conductive path between the primary particles even when the
charge-and-discharge cycle was repeated.
As shown in Examples 7, 8 and 10 to 13, when at least a part of the
surface of the primary particles was potassium phosphate or iron
phosphate, the rate characteristics tended to be more excellent and
the lifetime characteristics were also excellent. The reason for
this is considered to be that primary particles are strongly bound
by phosphate and iron and potassium contribute to enhancement of
lithium ion conductivity.
According to at least one embodiment and example described above,
an active material is provided. The active material includes a
primary particle containing a phosphorus-containing monoclinic
niobium-titanium composite oxide. The primary particle has a
concentration gradient in which a phosphorus concentration
increases from the gravity point of the primary particle toward the
surface of the primary particle. The active material can realize a
secondary battery capable of achieving excellent rate
characteristics.
While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to
limit the scope of the inventions. Indeed, the novel embodiments
described herein may be embodied in a variety of other forms;
furthermore, various omissions, substitutions and changes in the
form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
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